DEMONSTRATION

UNIT NAME
DEMONSTRATIONS

 
To demonstrate uniform motion in a straight line
It is rather difficult to demonstrate uniform motion of a freely
moving body due to the inherent force of friction. However, it is
possible to demonstrate uniform motion if a body of the forces
acting on it are balanced.
(a) Demonstration of uniform motion of a body in glycerine or caster
oil in a glass or a plastic tube
Take a glass or plastic tube one metre long and about 10 mm end
diameter. Close one end of it with a cork. Fill the tube with glycerine
(white) or castar oil upto the brim. Insert a steel ball or lead shot of
three mm diameter in it and close it with a cork such that no air
bubble is left in the tube. Take a wooden base 7.5 – 10.0 cm broad
having metallic brackets near its ends. Paint the board with white
paint or fix a sheet of white paper on it. Mount the tube on the wooden
base with the help of metallic brackets (to rest the tube like the base of
a fluorescent tube). Put marks on the base with black/blue paint or
ink at regular intervals of 10 cm each [Fig. D 1.1(a)]. To demonstrate

Fig. D 1.1: Demonstration of uniform rectilinear motion of a ball under

two balancing forces: (a) A demonstration apparatus 1 m
long (b) A low cost apparatus 50 cm long 219

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uniform motion keep the tube vertical and ask a student to note the
time taken by the ball to travel successive segments of 10 cm. Repeat
the experiment by inverting the tube a couple of times. It may be
emphasised that if a 10 cm segment is further sub-divided into
segments of 1 or 2 cm length, then the ball should travel successive
smaller segments also is equal intervals of time*.
This demonstration can also be done with a half metre long glass tube
and a half metre scale. It may be clamped vertical in a laboratory stand
[Fig. D 1.1(b)]. In this case students can also be asked to note the time
taken by the ball to travel successive segments of one cm.
The tube may be inclined slightly, say, at about 5° to the vertical. The
advantages of this are:
(i) The ball moves closer to the scale which reduces the parallax
error in observing its position on the scale.
(ii) The ball moving in contact with the wall of the tube is under
identical conditions throughout its motion. If you wish it to
move in the centre of the tube, i.e., along the axis of the tube,
then the vertical adjustment of the tube has to done with
greater precision.
In order to perform this demonstration with the half metre tube
more effectively, students may be encouraged to devise their own
mechanism to simultaneously record the distance moved by the ball
and the time taken to do so. For example, let one student watch the
falling ball at close distance and give signals by tapping the table as
the ball passes successive equidistant marks at a
pre-decided distance from each other.
100
A second student may start the stop-watch at the
90
sound of any tap. Thereafter, he observes and speaks
80
out the time shown by the watch at each successive
(cm)

70
tap, without stopping the watch. A third student may
60
keep noting the data of distance covered by the ball
50
40
and time elapsed since the measurement was started.
Ask students to plot the distance versus time graph
d

30
20
of the motion of the ball on the basis of this data and
10
discuss the nature of this graph [Fig. D 1.1(c)].
0 In this coordinated activity of three students, it is
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
likely that the first one may happen to miss giving
t (s)
signal at a mark when the ball passes it. He should
Fig. D 1.1(c): Distance–time graph for motion only indicate this by saying “missed” and a few
of metal ball in glycerine points less on the graph made with about 15 to 20
points are of no significance. Similarly, any tapping which he
subsequently feels, was not made at the right instant, he may indicate
* In this experiment, the ball accelerates for some time initially and approaches the
terminal velocity u0 according to relation u = u0 = (1-e-t/T). For a typical terminal
velocity u0 = 3 cms–1, the time constant T = 0.003s. Thus, the duration of accelerated
motion is so small that one may not at all bother for it.
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by saying “wrong”. Two students can also record this data, if there
is sufficient time between successive readings, the second one taking
over the task of the third. With some practice and by keeping the
watch in the left hand close to the ball, even one student can record
the data and take it up as an individual activity.
By mixing water with glycerine in a suitable ratio one can make
adjust the speed of motion of the ball such that it is neither too
slow as to cause boredom to the class nor so fast that the data is
difficult to record.
(b) By using a burette
The above demonstration may also be performed by using a long
burette. It has its own scale too. However, it may be difficult for
students sitting at the back in the classroom to see the scale. Also,
the upper end is open, which implies that several balls of the same
size should be available. In fact, in the demonstration (a) above, the
upper end of the tube may be kept open, if several balls of the same
size are available, since the most tricky part of it is to close the upper
end leaving no air bubbles inside the tube.
The demonstration with the burette can also be made more effective
in the same manner as discussed above.
Note:
1. In the class discussion following the demonstration of a steel
ball falling down with uniformed speed, an important question
will be “what are the two balancing forces under which it moves
with uniform velocity?” One is the net weight of the ball acting
downwards due to which its speed increases in the beginning.
As its speed increases, the resistance of liquid, acting upwards,
to the motion increases till it balances the weight. Then
onwards, the ball acquires terminal velocity and the speed
remains nearly constant.
2. There are a number of situations in everyday life where an object
falls down with uniform velocity in exactly the same manner as
the ball in a liquid.
(a) When a paratrooper descends from an aeroplane with the
help of a parachute, resistance of air on the parachute often
balances her/his weight. In such an event she/he moves
vertically down with uniform speed, except for some
horizontal drift due to the wind (Fig. D 1.2).
(b) Many children play with a toy parachute which is first thrown
up. Then it moves down in exactly the same manner as the
paratrooper with a parachute.
(c) A shuttle cock, which is used in the game of badminton,
may be shot vertically upwards, when it comes down,
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Fig. D 1.2: Discent of a parachute is nearly uniform

players often see that it is moving down with uniform

downward speed (if there is no wind) after a small initial
period of increasing speed.
3. This demonstration may also be done by the apparatus used for
finding the viscosity of liquid by Stoke’s law. However, for
demonstrating uniform motion in a straight line, the
demonstration is easier and better by: (a) using a scale to read
the position of the ball, and (b) keeping the tube slightly inclined
towards the horizontal.

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 
To demonstrate the nature of motion of a ball on an
inclined track
Make an inclined plane of about 50 cm length with 2 – 3 cm height at
the raised end. Alternately, one can use a double inclined track
apparatus and make the inclined plane by joining its two arms at the
base strip so that these form a single plane. Give it a low inclination
by raising one end of the base strip by about two cm with the help of
a wooden block, or a book, etc. (Fig. D 2.1). Now let a metronome
produce sound signals at intervals of ½ seconds. Keep the ball at the
higher end of one of the inclined planes. Release it at any signal (which
may be called 0th signal) and let students observe its position at 1st,
2nd, 3rd and 4th signals after the release. For this purpose, divide the
class into four groups. Explain to them in advance, with the help of a
diagram on the blackboard, that group I will observe the 1st position
of the ball, group II the 2nd position of the ball, and so on.

B′ A′

Fig. D 2.1: Motion of a ball on a double inclined plane

After the demonstration, there are as many observations for each

position of the ball as the number of students in each group. Let one
student in each group collect the observation in his/her group,
calculate the mean value and record it on the blackboard. Then it
can be shown that distances covered by the ball in successive intervals
of ½ second go on increasing by equal amounts when the ball roll
down the incline.
Note:
1. In the absence of a metronome, let a person tap on the table at a
steady pace which synchronises with extreme positions of the
pendulum of a clock, or a simple pendulum of 25 cm length on
a laboratory stand.
2. If a strobe-light is available, use it illuminate the ball moving
down the track. Then students can visually see successively
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 
To demonstrate that a centripetal force is necessary for moving
a body with a uniform speed along a circle, and that magnitude
of this force increases with angular speed
(a) Using a glass tube and slotted weights
Take a glass tube about 15 cm long and 10 mm outside diameter.
Make its ends smooth by heating them over a flame. Now pass a
strong silk or nylon thread about 1.5 m long through the tube. At
one end of the thread tie a packet of sand or a rubber stopper and
at the other a weight (W) (about three to 10 times the weight of the
sand/cork). First, demonstrate that on lifting the glass tube, the
weight stays on the table while the packet of sand or the stopper
gets lifts up (Fig. D 3.1).
Now by holding the glass tube firmly in one hand and the weight (W)
in the other, rotate the packet of sand in a horizontal circle. When the
speed of motion is sufficiently fast, the weight (W) can hang freely
without the support of your hand. Adjust the speed of rotation such
that the position of the weight (W) does not change. In this situation,
weight (W) provides the centripetal force necessary to keep the packet
or stopper moving along a circular path (Fig. D 3.2). If the speed of
motion is increased further, the weight (W) even moves up and vice
versa. Why?

Fig. D 3.1:The weight tied at the end passing

down the glass tube is much heavier
than the packet of sand
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As a safety precaution, in this demonstration, Packet moving

the packet to be rotated in a horizontal circle in a horizontal
should be a packet of sand, or a packet of a circle
few fine lead shots, or a rubber stopper, etc.,
lest it breaks off and strikes someone. Again,
the glass tube should be wrapped with two Glass tube
layers of tape, lest it breaks and hurt the hand held in hand
of the person demonstrating the experiment.
(b) Using a roller and a turn table
If a turn table (as you might have seen in a Nylon thread
gramophone) or a potter’s wheel is available,
it can also be used to demonstrate centripetal
force. A small roller is placed on the turn Weight
table and its frame is attached to the control
peg by a rubber band (Fig. D 3.3). The roller Fig. D 3.2: On revolving the packet of
is free to roll radially towards or away from sand at a suitable speed,
the centre. The disc is set in motion first at the weight lifts off the table;
the lowest speed of 16 revolution per minute. its weight is just enough to
The stretching of the rubber band indicates provide the necessary
centripetal force
that a force acts outwards along the radius.
At higher speeds, 33 r.p.m., or 45 r.p.m., or 78 r.p.m., the stretching
of the rubber band could seen to be larger and larger, showing that
greater and greater centripetal force comes into play. Note that as
the angular speed increases, the radius of circular motion of the
roller also increases due to elongation of the rubber band.

Turn table

Central peg

Roller
Rubber band

Fig. D 3.3: Elongation of rubber band indicates

that it is exerting a centripetal force
on the roller

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 
To demonstrate the principle of centrifuge

Bend a glass tube (about 10 to 15 mm diameter) slightly at its middle

to make an angle of, say, 160°. Fill it with coloured water leaving an
air bubble in it and then close its both ends with rubber stoppers.
Now mount it on the turn-table with both its arms inclined to
horizontal say, at, 10° while keeping the turn-table horizontal. The
lowest portion of the tube in the middle is attached to the central peg
of the turn-table (Fig. D 4.1). The air bubble then stays at the top of
one or both the arms of the glass tube.

Fig. D 4.1: A bent glass tube filled with a liquid but having an air bubble
attached to the central peg of turn table at its middle

Now rotate the turn-table and increase its speed in steps, 16 r.p.m.,
then 33 r.p.m., then 45 r.p.m. and then 78 r.p.m. As the speed of
rotation increases, draw attention that the air bubble is moving
towards the centre, the lowest part of the tube.
The rotating turn-table is an accelerated frame of reference. At every
point on it, the acceleration is directed towards the centre. Thus, an
object at rest in this frame of reference experiences an outward force.
Every molecule of water in the tube experiences this force, just like
the force of gravitation. Under the action of this force, denser matter
moves outwards and the less dense inwards.

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 
To demonstrate interconversion of potential and kinetic energy

Interconversion of kinetic and potential energies may be easily

demonstrated by Maxwell’s Wheel (Fig. D 5.1). It consists of a wheel
rigidly fixed on a long axle passing through its centre. It is suspended
by two threads of equal length, tied to the axle on two sides of the
wheel. In the lowest position of the wheel, separation between the
lower ends of the two threads is slightly more than that between
them at the supporting at the
top.
To set it in action the wheel is
rotated and moved up so that
both threads wind up on the Thread
axle. As the wheel moves up, it
gains some potential energy. On Thread
releasing, it moves down and its Axle
P.E. is converted to K.E. of
rotation of the wheel. At its
Wheel
lowest position when all the
length of the two threads has
unwound, all the energy of the
wheel is kinetic due to which the
threads start winding up in the
opposite direction.
Thus, the wheel starts moving
upwards, converting its K.E.
into P.E.
Fig. D 5.1: The Maxwell’s wheel

Note: In order to ensure that loss of energy in successive up and

down motions of the wheel be small, the threads should be
quite flexible, inextensible and identical to each other.

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 
To demonstrate conservation of momentum

The law of conservation of momentum can be demonstrated using

two bifilar pendulums of the same length using bobs of different
materials (Fig. D 6.1). The time period T for both pendulums is the
same. Initially the two bobs A and B touch each other in their rest
position. Also the suspension fibres of A and B are parallel to each
other in their rest positions.
The bob A is displaced with the
help of a wooden strip and allowed
to touch the reference peg C and
thus given a displacement, a,
which is noted with the scale. The
strip is then quickly removed, so
that bob A moves smoothly
towards the rest position and
collides with the bob B. The
maximum displacement a′ and b′
of the bobs A and B respectively
C AB
after collision are noted
simultaneously. On the right hand
side of B, a rider is put on the scale,
which is pushed by the ball B, as Fig. D 6.1: The bifilar pendulums
it undergoes the displacement b′.
Then reading the displacement of A directly and of B from the
displaced position of the rider becomes easier.
The masses mA and mB of the bobs are measured. The velocities of the
bobs, just before and just after the collision are proportional to their
displacements, since the time period, T, for both the pendulums is
equal and the velocity of a simple pendulum in its central position is
equal to (amplitude × 2π/T). Therefore, the equality of total momentum
of the two bobs before and after their collision implies
mA a = mA a′ + mB b′
Having measured a, a′ and b′, the above equality can be checked up
(a′ and b′ are the displacements’ after the impact).

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 
To demonstrate the effect of angle of launch on range of a
projectile
The variation in the range of a projectile with the angle of launch can
be demonstrated using a ballistic pistol or toy-gun and mounting it
suitably so that the angle of launch can be varied. While mounting
the gun care must be taken to see that the axis of the gun passes
through the centre of the circle graduated in degrees (Fig. D 7.1). If a
toy-gun is used, whose maximum range is more than the length of
the classroom, then this demonstration may be done in an open
area such as the school play ground.

Plumb-line

Clamp

90°

180°
Holes for
270° fixing the
clamp

Circular
protector
Wooden
circular disc
rigidly fixed
vertically

Fig. D 7.1: A set up to study the range of a

projectile fired with a toy pistol

As the gun is fired at different angles ranging between 0° and 90°,

the corresponding ranges are measured with care. A graph for the
angle of projection versus the range may be drawn.
Alternately one can also study the range of water jet projected at
different angles provided it is assured that water will be released at
same pressure.
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 
To demonstrate that the moment of inertia of a rod changes with
the change of position of a pair of equal weights attached to the rod

Take a glass rod and hang it horizontally from its centre of gravity with
the help of a light, thin wire. Take two lumps of equal mass of plasticine,
roll both of them separately to get discs of same size and uniform
thickness. Now attach them near the two ends of the rod (like rings) so
that the rod is again horizontal [Fig. D 8.1(a)]. Make sure that the
plasticine cylinders easily move along the rod. Give a small angular
displacement to the rod and note the time for 10 oscillations. Find the
time period for one oscillation. Now, move the rings of plasticine by
equal distances towards the centre of the rod so that it remains
horizontal [Fig. D 8.1(b)]. Give a small displacement to the rod and
again note the time period for 10 oscillations. Find the time period for
one oscillation. Are the two time periods the same or different? If you

Copper wire
Glass rod C.G.

Nut near Nut closer

the ends to centre

(a) (b)

Fig. D 8.1: Setup to demonstrate that total mass remaining constant, the
moment of inertia depends upon distribution of mass. Here
nuts have replaced the plasticine balls: (a) the movable mass
230 are far apart, (b) the masses are closer to the C.G. of the rod

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find that the time periods in both the situations are different, it shows
that the moment of inertia changes with the distribution of the mass
of a body even if the total mass remains the same.
An important caution for a convincing demonstration is that the
point where a thin metal wire is attached to the glass rod (the point
about which the glass rod makes rotatory oscillations) should remain
fixed. The metal wire should be so tied that the rod hangs horizontally
from it. It ensures that the axis of rotation passes through its C.G.
The wire can be fixed tightly by using a strong adhesive. Therefore,
the position of plasticine discs have to be adjusted so that the glass
rod hangs horizontally.

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  
To demonstrate the shape of capillary rise in a wedge-shaped gap
between two glass sheets

You would require two plane glass

slides, a thick rubber band, a match
Water level Match stick stick, a petri-dish, some potassium
between the slides permanganate granules and a felt-tip
Glass slides glass marking pen.
Rubber band
Coloured water Clean the two slides and the petri-dish
thoroughly with soap and water and
rinse with distilled water. Ensure that
no soap film remains on them. Fill the
dish about half with distilled water
coloured by potassium permangnate.
Tie one end of the pair of slides together
Petri-dish with a rubber band and put a match
Fig. D 9.1: Capillary rise of water is higher at the end stick between their free ends (Fig. D 9.1).
tied by rubber band in the wedge-shaped Dip this arrangement in the coloured
gap between the glass slides water in the petri-dish. Water rises more
at the tied end as compared to that at
the match stick end because the
separation between the glass slides
increases linearly from the tied end to
Capillary Water the match stick end.
tubes levels Coloured
water Note
Petri-dish

1. The same effect could be

demonstrated by using a number
of capillary tubes of different
diameters arranged side by side in
increasing order of diameter, as
shown in Fig. D 9.2.

Fig. D 9.2: Rise of water in capillary tubes of different 2. Students may take up this
diameters experiment as an activity or
project work.

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  
To demonstrate affect of atmospheric pressure by making partial
vacuum by condensing steam

To perform this demonstration you will need a round-bottom flask,

a glass tubing, a cork, cork borer, a long piece of pressure rubber
tube just fitting the glass tubing, a pinch cock, burner, tripot stand,
laboratory stand with a clamp and large water container.
Take some water in a round bottom flask. Close its mouth tightly
with a rubber cork, in which a short glass tube is fitted. Attach a
pressure rubber tube, about 1.5 m long, in the open end of the glass
tube. Heat the water, as shown in Fig. D 10.1(a). The steam produced
in the flask expels the air from the flask, the glass tube and the
rubber tube. Stop heating after some time and tightly close the mouth
of the rubber tube with a pinch cock immediately.
Invert the flask and clamp it as high as possible in a tall stand placed
on the table [Fig. D 10.1(b)]. Dip the free end of the rubber tube in
coloured water kept in the large container on the ground and release
the pinch cock. As the flask cools, water from the container rushes
through the glass tube into the flask. The students will naturally

Rubber tube

Short glass tube

Rubber cork

Water

Fig. D 10.1 (a): On heating the water in flask,

steam drives air out from it

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Fig. D 10.1 (b): Atmospheric pressure pushes coloured water up

into the flask as steam in the flask condenses

become curious to know the reason why water rises through the height.
It may be explained in terms of difference in pressure of air on the
surface of the water in the container and inside the flask.
Note
To make this experiment more spectacular, a student may climb
on the table and raise the stand by another 2 m. Then the pressure
rubber tube may also have to be longer.

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 
To study variation of volume of a gas with its pressure at constant
temperature with a doctor’s syringe

This demonstration can be given with the help of a large (50 mL or

more) doctor’s syringe (disposable type), laboratory stand, grease or
thick lubricating oil, 200 gram to 1 kg weights which fit over one
another, cycle value-tube, rubber band, a wooden block and a
laboratory stand.
Make the piston in the syringe air tight by applying a drop of thick
lubricating oil or grease into the syringe. Draw out the piston in the
syringe so that the volume of air enclosed by it is equal to its full
capacity. Next close the outlet tube of the syringe by fixing a piece of
cycle value-tube on it and folding the valve-tube. Hold the syringe
vertically with a laboratory stand with its base resting on a wooden
block (Fig. D 11.1).
Press the piston downward with the hand to compress the air inside.
Release the piston and observe, whether the air inside regains its
initial volume by pushing the piston up. Since, the friction between
the piston and the inner surface of the syringe is quite large, both

Piston

40
35

Compressed air 25
20
15
Rubber or Graduated
10
cloth pad 5
outer body
0 of syringe
Wooden block Cycle valve
tube

Fig. D 11.1: The load is kept on the piston of the syringe to apply
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being of plastic, the air inside is unable to push the piston upto its
original position. When the piston comes to rest, the thrust of
atmospheric pressure plus limiting friction is acting on it downwards.
Note the volume of enclosed air in this position of the piston.
Next, pull the piston up a little and release. Again it does not reach
quite upto its original position. This time the thrust of atmospheric
pressure minus limiting friction is acting on it downwards. Note this
volume of air also and check that the mean of the two volumes so
measured is equal to the original volume of air at atmospheric pressure.
Now balance a 1 kg weight on the handle of the piston. Note the two
volumes of enclosed air, (i) piston slowly moving up and coming to
rest, and (ii) piston slowly moving down and coming to rest and find
their mean. In this manner note volume, V, of air for at least two
different loads, say 1 kg and 1.8 kg, balanced turn by turn on the
piston. Check up, in the end that volume for no load is same as that
at the beginning to ensure that no air leaks out during the experiment.
Draw a graph between 1/V and load W for the three observations,
W = 0 kg, 1 kg and 1.8 kg if a graph black-board is available.
Alternately, it may be given as an assignment to students.

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 
To demonstrate Bernoulli’s theorem with simple illustrations

(a) Suspend two simple pendulums from a horizontal rod clamped

to a laboratory stand (Fig. D 12.1). Use paper balls or table tennis
balls as bobs. Their bobs should be close to each other and at
the same height but not touching each
other. Ask the students what would
happen if you strongly blow into the
space between the bobs. A person/
student not thinking in terms of
Bernoulli’s theorem would conclude that
air pushed into this space will push the
bobs away from each other. Now blow air
between the two bobs suspended close
to each other and ask them to observe
what happens. The speed of air passing
between them gets increased due to less
space available and so the pressure there,
gets decreased. Thus, the pressure of air
on their outer faces of the bobs pushes Fig. D 12.1
them closer. That is why one observes
the bobs to actually move closer.

(b) Place a sheet of paper supported by

two books in the form of a bridge.
Let the books be slightly converging
(Fig. D 12.2) i.e., their separation is larger
on the side facing you. Now, you blow
under the `bridge’, the paper `bridge’ is
pushed down.

(c) Hold the shorter edge of a sheet of paper

horizontally, so that its length curves
down by its weight [Fig. D 12.3(a)]. If you
press down lightly on the horizontal part
of the curve with your finger the paper Fig. D 12.2
curves down more. Now, instead of
touching with the hand hold the horizontal edge of the sheet of
paper close to your mouth. Blow over the paper along the
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Sheet of
Paper Blown Air

Fig. D 12.3 (a) Fig. D 12.3 (b)

lifts up [Fig. D 12.3(b)]? The curved shape of paper makes the

tubes of flow of the wind narrower as the wind moves ahead as
shown in [Fig. D 12.3(c)]. Thereby its speed increases and pressure
on the upper side of the paper decreases.
(d) Fill coloured water in an insecticide/pesticide spray
pump. Spray the water on a white sheet of paper.
Coloured drops deposit on the paper. It is evident
that water from the tank rises up in the tube
attached to it and is then forced ahead in the form
of tiny droplets. But what makes it rise up in the
tube? As the pump forces air out of a fine hole, the
speed of air in the region immediately above the
open end of the tube in the tank becomes high
(Fig. D 12.4). Thus, the pressure of air in the region
is lower than the surrounding still air (which is equal
to atmospheric pressure). Right in this region, just below
Fig. D 12.3 (c) the hole in the pump is the upper end of the fine tube
through which water rises up, due to atmospheric
pressure acting on the surface of water outside the tube.

Fig. D 12.4

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(e) Fig. D 12.5 shows the construction of a Bunsen burner. The fuel
gas issues out of the jet J in the centre of the vertical tube. Due
to the high speed of gas, its pressure gets lowered. So, through a
wide opening in the side of the vertical tube air rushes in, mixes
up with fuel gas and the gas burns with a hot and blue flame. If
the air does not get mixed with fuel gas at this stage and comes
into contact with it only at the flame level, the flame will be
bright yellow-orange like that of a candle, due to incomplete
combustion of the gas which gives off comparatively less heat
than when it burns with a blue flame.

Fig. D 12.5

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 
To demonstrate the expansion of a metal wire on heating

Stretch a length of any metal wire firmly between two laboratory

stands, which are fixed rigidly on the table by G-clamps (Fig. D 13.1).
Suspend a small weight at the centre of the wire and stretch the wire
as tightly as possible, without significantly bending the iron stands.
However, the wire cannot be made straight and some sagging is
inevitable due to the weight suspended at the centre. Place a pointer
on the hind side of the upper edge of the weight to serve as reference.
Heat the wire along its entire length by a spirit lamp or a candle. The
wire is seen to sag more and the weight moves down. Remove the
flame to let the wire cool. As the wire gradually cools, the weight
ascends to its original position.
Wire

Pointer

Small
weight

Fig. D 13.1: A taut wire sags on heating due to its thermal expansion

Note:
The wire can also be heated electrically, if so desired. Use a step-down
transformer which gives various voltages in steps from 2 volt to 12
volt. The advantage is that heating of the wire for a certain voltage
applied across it will be uniform along its whole length and the
observed sagging by this heating will be repeatable.
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 
To demonstrate that heat capacities of equal masses of
aluminium, iron, copper and lead are different

This demonstration can be performed with

four cylinders of aluminium, iron, copper
and lead having equal mass and cross-
sectional area, a rectangular blocks of
paraffin wax, beaker/metallic vessel,
thread, water and a heating device.
Since the four solid cylinders are having
equal mass and equal cross-sectional
area, their lengths are inversely
proportional to their densities. Take
water in a beaker or a metallic vessel and
boil it. Suspend the four cylinders, tied
with threads, fully inside boiling water
(cautiously, if a beaker is being used).
After a few minutes all have attained
the temperature of boiling water
[Fig. D 14.1(a)].
(a)
Take out the cylinders in quick
succession and place them side by
side on a thick block of paraffin wax
[Fig. D 14.1(b)]. The cylinders sink to
different depths in the paraffin wax. They
all cool from temperature of boiling water
to melting point of wax during the process
of sinking. Although all the cylinders have
the same mass, but the amount of heat
they give out are different.
An alternative (and more convenient to do) (b)
method is to have a wooden board with
semi cylindrical grooves resting against a Fig. D 14.1: Qualitative comparison of
block. Equal length of each groove is heat capacities of different
metals
initially filled with wax. Hot cylinders are
placed on this wax in the grooves, instead
of on the wax block.
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Note
A substantial portion of heat given out by each cylinder is
radiated into the atmosphere. Moreover, they radiate at
different rates because of the difference in their surface areas.
Therefore, by this experiment we only get a qualitative
comparison of the heat capacities of these solids.

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 
To demonstrate free oscillations of different vibrating systems

A number of demonstrations involving vibrating systems are

presented through (a) to (j). Demonstrate as many vibrating systems
as possible and discuss the following in each case:
(i) What are the energy changes that occur during vibrations?
(ii) How can the frequency of vibration be altered?
(iii) Can the damping of the system be reduced? If so, how?
(iv) How does the force acting on the oscillating body vary with
its displacement from the
mean position?
(a) Simple pendulum : Make a
rather long and heavy simple
pendulum following the steps
described in Experiment 6. One
may tie a brick or a 1kg weight
at one end of a strong thread
about 1.5 m in length. Suspend
the pendulum from a stand
having a heavy base so that it
does not topple over. The base
can be made heavy by putting
a heavy load on it, say a few
bricks. Alternatively, the stand
may be clamped on the table
with a G-clamp. The vertical rod
of the stand may be further
supported by tying it to three
G-clamps fixed on the table
(Fig. D 15.1). A sturdy stand will
Fig. D 15.1: Set up to study oscillations
help in keeping the pendulum
of a heavy pendulum
oscillating for quite a long time
with very small damping.
(b) Vibrating hacksaw blade: Clamp a hacksaw blade (or a thin metal
strip) with its flat surface horizontal at the edge of the table by a
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G-clamp (Fig. D 15.2). Load the free end by

about 20 g of plasticine or by putting a 20 g
weight on the flat free end and fastening it to
the blade with a thread. Let the free end of the
blade vibrate up and down. Repeat the
demonstration with a smaller load and then
with no load on the blade. Compare the
oscillations with different loads.
(c) Oscillating liquid column: Fix a U-tube of
large diameter (about 2cm) on a stand with
Fig. D 15.2: A hacksaw blade clamped at its arms vertical. Fill liquid of low viscosity
one end vibrates up and down
e.g., water or kerosene or methylated spirit
in it. Let the liquid column oscillate up and
down in the tube (Fig. D 15.3). For this
purpose blow repeatedly into one arm of
the U-tube with your mouth as soon as
the liquid column in the arm you are
blowing attains maximum height so as to
generate a small air pressure in it each
time so as to oscillate the liquid column
by resonance. Another method is to slightly
tilt the stand to one side repeatedly, with
the U-tube fixed on it so as to oscillate the
liquid column by resonance.
A low cost U-tube can be improvised with
two straight tubes of about 3.5 cm to 4 cm
Fig. D 15.3: Set up to demonstrate oscillations diameter and each of length about 50 cm. Fix
of liquid column in a U-tube the tubes vertically on a wooden board about
20 cm to 30 cm apart. Join their lower ends
with a piece of a rubber tube, or a piece of hose
pipe made of plastic. A plastic hose pipe is better
because it bends to the U-shape easily. Fill this U-tube with
coloured water upto about 10 cm below the two open ends.
Oscillate the liquid in the tube by either of the two methods
described above.
(d) Helical spring : Attach a suitable mass, say 1kg, at one end of a
helical spring (Fig. D 15.4). Suspend the spring vertically. Pull
the weight down through a small distance and let it go. Observe
and study the vertical oscillations of the mass suspended by the
lower end of the spring.
(e) Oscillations of a floating test-tube : Take a test tube and fill at its
Fig. D 15.4: A load attached bottom about 10 g of lead shots or iron filings or sand. Float the
to the lower end of a helical tube in water and adjust the load (lead shots or iron filings or
spring oscillates up and down sand) in the tube till it floats vertically. Keeping the tube vertical
push it a little downwards and release it so that it begins to
oscillates up and down on the surface of the water (Fig. D 15.5).
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(f) Oscillations of a ball along a curve : Take about

30 cm length of aluminium curtain channel
and bend it into an arc of a circle. Put it on a
table and provide it proper support by two
rectangular pieces of thick card board or
Lead shots
plywood to keep it standing in a vertical plane.
Let a ball–bearing or a glass marble oscillate
in its groove (Fig. D 15.6). Alternatively place
a concave mirror (10 cm or 15 cm aperture)
or a bowl or a karahi on a table with its
Fig. D 15.5: A test tube floating
concave side facing up. Let a ball bearing or a in vertical position
glass marble oscillate in it along an arc passing due to a load in it,
through its lowest point as shown by point P oscillates up and
in Fig. D 15.6. down, when it is
pushed a little and
(g) Oscillations of a ball on the double inclined then released
track: Adjust a double inclined
track on a table with its arms
equally inclined to the horizontal
(Fig. D 15.7). Release a steel ball–
bearing (2.5 cm diameter) from
the upper end of one of the arms
and let it oscillate to and fro
between the two arms of the
double inclined plane.
Fig. D 15.6: Arrangement to demonstrate the
(h) Oscillations of a trolley held to-and-fro motion of a steel ball in
between two springs on a table: a channel in the form of an arc of
Take a trolley and attach two a circle
identical helical springs at each of
its ends such that the springs are along a straight line. Place the
trolley on a table and fix the free ends of the springs to two rigid
supports on opposite ends of the table so that the springs are
under tension along the same straight line [Fig. D 15.8(a)].
Displace the trolley slightly to
one side keeping both springs
under tension. Release the Enlarge view of
trolley and observe its to and central curved part
fro motion along the length of
the springs. Find the time
period of oscillations and also
make a note of damping.
(i) Oscillations of a trolley
attached to a spring: Remove
Fig. D 15.7: Arrangement to demonstrate the to-
one of the springs from the set and-fro motion of a ball along a
up arranged for demonstration double inclined track
(h) shown in Fig D 15.8 (a).
Displace the trolley to one side and release it. Compare the time
period of oscillations affect of damping with the earlier case.
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(a)

Wheels not
touching
the table
(b)

Fig. D 15.8: (a) Set up for demonstrating the to-and-fro motion

of a trolley held between two identical springs
(b) Arrangement to demonstrate the to-and-fro motion
of a trolley suspended from a high support while it is
held between two springs on either side

(j) Oscillations of a trolley suspended from a point and held between

two springs: Set up the trolley with two springs on a table as
described in demonstration (h) above. Attach an inflexible string
to the trolley as shown in Fig. D 15.8(b). Fix the other end of the
string to a stand kept on a stool placed on the table or to a hook
on the ceiling such that the trolley remains suspended just above
the table. Set the trolley in oscillation by displacing it slightly to
one side. Study how the time period of oscillations and damping
get affected as compared to the case when the trolley was placed
on the table, as in demonstration (h).

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 
To demonstrate resonance with a set of coupled pendulums

Take two iron stands and keep them on the table at about 40 cm
from each other. Tie a half metre scale (or still better a straight strip
of wood about 1.5 cm wide) between them so that it is horizontal
with its face vertical and free to rotate about its upper edge
(Fig. D 16.1). Near one edge of the scale suspend a pendulum with a
heavy bob (say, approximately 200 g). Also suspend four or five
pendulums of different lenghts with bobs of relatively lower masses.
However, one of them should be exactly of the same length as the
one with the heavy bob, as described.

Fig. D 16.1: A set up to demonstrate resonance

Let all the pendulums come to a rest after setting up the arrangement
described above. Gently pull the bob of the heavy pendulum and
release it so that it starts oscillating. Make sure that other pendulums
are not disturbed in the process. Observe the motion of other
pendulums. Which of the pendulums oscillates with the same
frequency as that of the heavy pendulum? How does the amplitude
of vibrations of different pendulums differ?

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 
To demonstrate damping of a pendulum due to resistance
of the medium

(a) Damping of two pendulums of equal mass due to air: Set up two
simple pendulums of equal length. The bob of one should be small
in size say made of solid brass. The bob of the other should be of
the same mass but larger in size — either of a lighter material like
thermocole or a hollow sphere. Give them the same initial
displacement and release simultaneously. Observe that in the
pendulum with the larger bob the amplitude decreases more
rapidly. Due to its larger area, air offers more resistance to its
motion. Though both pendulums had the same energy to start
with, the larger bob looses more energy in each oscillation.
(b) Alternative demonstration by comparing damping due to air and
water: Set up a simple pendulum about half metre long with a
metal bob of 25 mm or more diameter. In its vertical position the
bob should be about 4 cm to 5 cm above the table. First, let the
pendulum oscillate in air and observe its damping. Now place a
trough below the bob containing water just enough to immerse
the bob in water. Let the pendulum oscillate with the bob immersed
in water and note the effect of changing the medium on damping.

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 
To demonstrate longitudinal and transverse waves

A few characteristic properties of

transverse and longitudinal waves can
be demonstrated with the help of a
slinky, which is a soft spring made of a
thin flat strip of steel (about 150 to 200
turns) having a diameter of about 6 cm
and width 8 cm to 10 cm. Nowadays
slinky shaped spirings made of plastics
are also available. Let two students hold
each end of the slinky and stretch it to
its full length (at least 5 metres) on a
smooth floor. Give a sharp transverse
jerk at one end and let the student
observe the pulse as it moves along the
spring [Fig. D 18.1(a)].
Find the speed of the pulse by
measuring the time taken by it to move (a)
from one end to the other along the
stretched length of the spring. For more Fig. D 18.1(a): Motion of a pulse through a
accuracy, instead of measuring time slinky
taken by the pulse to move from one
end to the other, measure the time taken by it to make three to four
journeys along the entire length of the spring. This would be possible
because each pulse moves back and forth along the spring a few
times before it dies.
Repeat the experiment by decreasing
the tension in the spring (by stretching
it to a smaller length) and find the speed
(b)
of the pulse. Does the speed depend on
tension? Fig. D 18.1(b): A compression moving along
the length of a slinky
The slinky can also be used to
demonstrate propagation of longitudinal waves. To do so, give a
longitudinal jerk at one end of the slinky, keeping the slinkly
stretched on the floor to about half the length (2.5 m) than while
demonstrating movement of a transverse pulse [Fig. D 18.1(b)]. Ask
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the students to observe the motion of the pulse in the form of

compression of the spring.
The damping may be too high if the floor is not very smooth. In that
case the experiment may be performed by suspending the slinky from
a steel wire stretched between two pegs firmly fixed on opposite walls
of the room. In order to minimise the effect of sagging of the spring in
the middle, support the spring by tying it to the wire with pieces of
thread spaced at about 25 cm from each other. All pieces of thread
must be equal in length.
The transverse waves may also be demonstrated with the help of a
flexible clothes line or a rubber tubing or a rope instead of a slinky.
Tie one end of the rubber tubing or the clothes line to the knob of a
door and give it a jerk at the other end while keeping it stretched. If
the rubber tube is heavy (fill water in it) and is held loosely, the pulse
would move slowly to make better observation.
Instead of a single pulse, a series of pulses one after the other creating
an impression of a continuous wave propagation may also be
demonstrated. This can be done by using a slinky or a flexible clothes
line. Stretch the slinky on the ground and ask one of the students to
hold one end firmly. Instead of giving just one jerk at the other end,
move the hand to and fro continuously to make waves of wavelength
about 0.5m which can be seen to move continuously along the spring.

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 

To demonstrate reflection and transmission of waves at the

boundary of two media

Stretch the slinky on a smooth floor

or suspend it from a stretched steel
wire as described in Demonstration
18.1. Keeping one end fixed, send a
pulse from the other end. Note the
size and direction of displacement of
pulse before and after it gets reflected
at the fixed end. Note that the
reflected pulse is upside down with
little change in its size in comparison
to the incident pulse [Fig. D 19.1(a)].

Next join the coil spring (slinky) with

another long helical spring of heavier
mass end to end [Fig. D 19.1(b)].
Stretch them by holding the free end
of each spring and produce a pulse
at the free end of the lighter spring
(slinky). Observe what happens when
(a)
the pulse arrives at the joint of two
springs. In what way (i.e., with Fig. D 19.1 (a): A pulse reflected at a fixed end
respect to size and direction of undergoes phase change of π
displacement) does the reflected
pulse undergo a change? Does the
pulse transmitted to the heavier spring also undergo any change?

Repeat the demonstration by sending the pulse from the end of the
heavier spring. Note how the reflected and transmitted pulse undergo
a change at the boundary of the two springs as compared to the
incident pulse [Fig. D 19.1(c)].

How do these changes differ from those in case of incident pulse

going from lighter to heavier spring?
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Fig. D 19.1 (b): Reflection and transmission Fig. D 19.1 (c): Reflection and transmission of
of a pulse moving from a pulse moving from a denser
a rarer medium to a medium to a rarer medium
denser medium

Now join the slinky (coil spring) to a fine thread instead of a heavier
spring. Stretch the spring and the thread and produce a pulse at the
free end of the spring. Note what happens to the pulse at the boundary
of the spring and the thread.

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 
To demonstrate the phenomenon of beats due to
superposition of waves produced by two tuning forks of
slightly different frequencies

Take two tuning forks of identical frequency. Attach a small piece of

plasticine or wax to the prongs of one of the tuning forks. This will
slightly lower the frequency of the tuning fork. Now holding them
one in each hand strike both the tuning forks simultaneously on two
rubber pads. Place them close to each other.
Carefully listen to the combined sound produced by the two tuning
forks. Gradual increase and decrease in the intensity of sound will
be heard. It is due to beats produced by the superposition of waves
of slightly different frequencies. You can also count the number of
beats produced per second if their frequency does not exceed two or
three beats per second. The person who is listening to the beats,
gives a silent signal at each minimum intensity or maximum intensity,
e.g., by shaking his head in the manner we say ‘yes’. Then a second
person with a stop-watch, either finds the time taken by 10 beats or
counts the number of beats in 5 seconds. The person with the stop-
watch will also listen to the beats, though less loudly and may measure
the frequency without the aid of a signal by the first person.
If two tall tuning forks of the same frequency mounted on resonating
wooden boxes are available, all the students in a classroom may be
able to listen to the beats. Place them on a desk in the centre of the
classroom. Let there be pin-drop silence in the classroom. Then strike
the tuning forks with a rubber hammer in quick succession, with
roughly equal force. Make their frequencies slightly different by
loading one with plasticine or wax or by tightly attaching a small
load with adhesive tape. Both tuning forks must be of rather good
quality and must give audible sound for about 8 to 10 seconds in
spite of dissipation of energy in the resonating box.

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 
To demonstrate standing waves with a spring

Stretch the wire spring (heavier one and not the slinky) to a length of
6 m to 7 m, by tying its one end to a door handle. It may sag in the
middle but that will not affect the demonstration. Give a transverse
horizontal jerk at the free end, a pulse will travel along the spring,
and get reflected back and forth. If instead of stretching the spring in
air it is stretched along the ground, then due to large damping, the
results will not be so clear and convincing.
Now generate a continuous transverse wave in the spring by giving
series of jerks to the spring at fixed time intervals. Change the
frequency of the waves by changing the time period of oscillating your
hand till stationary waves are set up. You will find that stationary
waves are produced only when an integral number of loops, i.e., 1,2,3
etc. are accommodated in the entire length of the spring. In other
words, stationary waves are produced corresponding to only some
definite time periods.
Ask one of the students to measure the time period of standing waves
when one loop, two loops, three loops, and so on are formed in a
given length of stretched spring. For the same extension of the spring,
and thus for the same tension in the spring, how are the time periods
of stationary waves of one loop, two loops, and three loops related to
each other?
While producing stationary waves, suddenly stop moving your hand
to and fro and thus stop supplying energy to the spring. This is best
done by taking the help of a stool on which your hand rests while
producing the waves as well as when you stop your hand. Observe
that the spring continues to vibrate for some time with the same time
period and the same number of loops. Thus, it can be demonstrated
that the stretched spring is capable of making free oscillations in
several modes—with one loop, two loops, three loops, etc. The various
time periods with which you can produce stationary waves in it, are
also the natural time periods of the spring.
Thus, when you are producing and observing stationary waves in the
stretched spring, you can consider it as a resonance phenomenon.
However, in this case, the object being subjected to forced oscillations
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the several time periods, unlike the simple pendulums with which
you experimented earlier to study the phenomenon of resonance.
One can also demonstrate stationary waves with a spring when its
both ends are free to move. Tie a thread, 3 – 4 m in length, at one
end of the spring. Tie other end of the thread to a hook on the wall or
a door handle. Stretch the spring by holding it at its free end and
send a continuous transverse wave in the spring by moving the end
in your hand. Do you observe that the stationary waves now produced
are somewhat different than those produced when one end of the
spring was fixed. Note the difference in the pattern of stationary
waves in the two situations and discuss the reason for the difference.
Also ask to note the number of loops produced when a stationary
wave is set in the spring.
Change the time period of the wave by adjusting to and fro motion of
your hand to produce ½ loop, 1½ loop, 2½ loop and so on for same
extension of the spring.
How are these time periods related to the various time periods of
vibration when the end not in your hand was kept fixed and extension
of the spring was the same?
Note
Mathematically, it can be shown that superposition of two waves
of the same frequency (and thus moving with same velocity)
travelling in opposite directions in an infinite medium, produce
stationary waves. In this mathematical treatment, there is no
need of specific frequencies at which the stationary waves are
produced. However, it is not possible to translate that
mathematical result into a simple experimental demonstration.
In an experiment we have to take a finite medium, like the
stretched spring of finite length. A finite medium with boundaries
has its natural frequencies and thus experiment is done at those
frequencies. In the above demonstrations one wave is produced
by hand and the other (travelling in the opposite direction) is
the reflected wave and their superposition produces stationary
waves, exemplifying the above referred mathematical result.

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