Unit #3 Forces Physics O-Level

Section-1 = Forces

Force:

It is pull or push on an object that changes or tends to change the state of rest or uniform motion of that object.

Effect of Force:

 Force can change the shape of a body.

 Force can stop the moving body.
 Force can set the body into motion.
 Force can accelerate the body.
 Force can decelerate the body.
 Force can change the direction of a moving body.

Types of Forces:

Forces Description
Force of Gravity The pull of earth acting on an object.
Force of gravitation It is force of attraction between any two
objects in the universe.
Tension It is the force experienced by stretched or
compressed objects.
Friction The contact force which opposes the
motion of body, due to the roughness of
surface.
Resistance When a body is dragged through a fluid
(air or liquid), there is a friction between
body and fluid called as resistance or
viscous force.
Contact force or normal reaction force When an object is made in contact with a
surface, then the reaction force of that
surface is called contact force or normal
reaction force.
Electric force The push or pull between electric
charges.
Magnetic force The push or pull between magnets.

Effects of Friction:
Friction is a contact force that slows down moving objects. Friction has both positive and negative effects.

Negative Effects Positive Effects

Force f friction causes wear and tear in Force of friction helps in holding the
moving parts of machine. objects, walking on ground
Force of friction reduces the engine Force of friction helps in stopping the
power moving vehicles

Methods of Reducing Friction

1. Using highly polished surface for moving parts.

2. Using a layer of lubricants between moving parts.
3. Using ball bearings to enable surface to roll over.
4. Making the aerodynamic shapes of moving objects.

Effects of friction on the motion of a vehicle

1. Tyre surface: If tyre surface is in good condition then there is more friction between the tyre and road. The
moving vehicle can be stopped easily within the stoppi9ng distance.

2. Road Condition: If road is wet, the friction between tyres and road reduces, resulting to increase in stopping
distance. The vehicles can also skid at turns, due to wetness of road.

3. Braking Force: If braking pads/discs are in good condition, then braking force causes more friction and
stopping distance reduces.

Braking, thinking & stopping distances

1. Braking distance: The distance travelled by a moving vehicle during the time that the brakes are applied.

2. Thinking distance: The distance travelled by moving vehicle: during the reaction time of driver, before
applying the brakes.

3. Stopping distance: The total distance travelled by moving vehicle, between thinking the distance and
stopping the vehicle. i.e. Stopping distance = thinking distance + breaking distance.

The braking, thinking and stopping distances are not equal due to following factors.

a. The road condition

b. The tyres condition
c. The brakes condition
d. The speed of moving vehicle
e. The vehicle is loaded or unloaded.
f. The human reaction of driver
Circular motion

If the distance of an object remains constant from a fixed point, throughout its motion, then object is in circular
motion. The circular & motion has following characteristics.

 If speed of object in circle is constant, its direction keeps on changing, so velocity is not constant.
 The direction of velocity at any instant in circular motion is determined by the tangent to circle at that
point.
 The force which keeps the object moving circular path is called centripetal force. This force is always
directed towards centre of circle.

Examples of circular motion:

1. Motion of electrons:

The electrostatic fore exerted by the nucleus on electron, provide centripetal force to electrons. The electrons
keep on orbiting around the nucleus in circular motion.

2. Motion of Satellite:

The force of gravity of earth provides centripetal force to the satellite. The satellite keeps on orbiting around the
earth in circular motion.

3. Motion of Planets:

The planets move around the sun due to gravitational force. The gravitational force provides centripetal force to
the planets. Each planet in the solar system is directed towards sun, due to centripetal force and keeps on
moving in fixed orbit.

Balanced and Unbalanced Forces:

Balanced Forces: Two equal forces acting in opposite directions cancel each other out. These forces are
balanced forces. The balanced forces produce two effects.

1. Object is either at rest or

2. Moving at a steady speed.

Unbalanced Forces: Two forces of different values acting in opposite directions, on an object are unbalanced
forces. The unbalanced forces

1. Either accelerate the object or

2. Decelerate the object

Newton’s law of motion:

First Law: A body continues its state of rest or uniform motion until an external force acts on it.
Second Law: When a force acts on a body, then acceleration is produced such that

i. a ℘ F i.e. acceleration directly proportional to the force applied.

ii. a ℘ 1/rn i.e. acceleration inversely proportional to the mass of body.

The acceleration, force and mass are related by the formula:

The unit of force is Newton (N).

Third law: To every action there is equal and opposite reaction.

SECTION —2

Scalar and Vector

1. A scalar quantity is that which has magnitude only.

2. A vector quantity has magnitude as well as direction.

Examples:

Quantity Scalar Vector

Distance
Displacement
Length
Speed
Velocity
Time
Acceleration
Force
Weight
Density
Area
Volume
Energy
Pressure

Resultant Vector: The combined effect of two vectors is called resultant vector.

Methods to determine resultant vector:

1. By addition: If two vectors are in the same direction then their magnitudes are added up to find resultant
vector.

2. By Subtractions: If two vectors opposite direction, then they are subtracted from each other to find resultant
vector.

3. Parallelogram method: If two vectors are at certain angle, then Parallelogram method is used to find
resultant vector. “Complete the parallelogram joining the given vectors at their angle. The length of diagonal
determines the resultant vector.”

4. Triangle method:

If two vectors art at 900, then the resultant vector is obtained by head to tail rue.

“Join the head of first vector with the tail of second vector. The resultant vector is
obtained by joining the tail with the tail of first vector and head with the head of
second vector.”

SECTION — 3
Deformation

Elasticity or Elastic deformation:

When a force is applied then shape of an object can be changed. On releasing the force if object regains its
original shape then this effect is called elasticity or elastic deformation.

Elastic limit or limit of proportionality:

It is the maximum extension in an elastic object, after which it either breaks or deforms permanently.
Hooke’s Law:

Within elastic limit, the extension produced in an elastic object is directly proportional to the force applied.

Point A = Hooke’s law is valid

Point B = Elastic limit or limit of proportionality

Point C = Breaking point

A to B = Elastic deformation

Experiment to verify Hook’s law

Apparatus: Spring, metre rule, stand, weights or loads. Arrangement & Procedure:

i. Attach a spring with stand and measure its original length (l1) with rule.

ii. Attach a load or weight at the end of spring. Measure stretched length of spring (l2) with rule.

iii. Similarly, attach different loads at the end of spring and measure the stretched lengths of spring for each
load.

iv. Record the results in the table.

No. of Obs. Original length (l1) Final length (l2) Extension (l2-l1) Load/weight
 1
2
3
4
5

v. Draw the graph between extension and load the graph is a straight line, passing through origin. So Hooke’s
law is verified.

Unit #5 Turning Effect of Forces Physics O-Level

Turning Effect:

When a force is applied then object may turn round a fixed point. This is called turning effect of force. The
point around which objet turns is called pivot or hinge or fulcrum.

The turning effect depends on

a. Magnitude of force (F) applied.

b. The perpendicular distanced (d) from the line of action of force to the pivot.

Example of Turning Effect:

a. Opening of a hinged door.

b. Paddling of a bicycle.
c. Opening of a bottle cap.
d. Turning the steering wheel.

Moment of Force:

“It is the product of force and perpendicular distance from the line of action of the force to the pivot.”

Formula:

Where F = magnitude of force

d= perpendicular distance

Unit: nm.

Clockwise and Anticlockwise Moments:

  If the object turns clockwise, then turning effect is called clockwise moments.
 If object turns anticlockwise effect is called anticlockwise moments

Equilibrium:

“A body at rest moving with uniform velocity is said to be in equilibrium.”

The two condition of equilibrium are:

1. Sum of forces in one direction is equal to sum of forces in other direction.

2. Sum of clockwise moments is equal to sum of anticlockwise moments.

Principle of moments:

For an object in equilibrium, the sum of clockwise moments is equal to sum of anticlockwise moments. i.e.

Clockwise moments = anticlockwise moments

i.e. F1xd1= F2xd2

Experiment: To investigate the Principle of Moments

Apparatus: Uniform metre rule, load (W1), load (W2), strings, knife edge, retort stand.

Procedure:

1) Set up the apparatus as shown in Figure with the knife edge at the 50 cm mark.
2) Balance the system by adjusting the distances d1 and d2.

3) Vary d1 and change d2 so that the system is balanced for 5 sets of d1 and d2.

4) Calculate the anticlockwise moments W1 x d1 and the clockwise moments W2 x d2 and tabulate as follows:

Table

W1 D1 W2 D2 W1 x d1 W2 x d2

Observation:

From the table, the anticlockwise moments given by (W 1 x d1) are found to be equal to the clockwise moments
given by (W2 x d2) for each set of d1 and d2.So principle of moments is verified.

Centre of mass (c.m) or Centre of gravity (c.g)

The point through which whole mass or weight of an object appears to act is called its centre of mass (c.m) or
centre of gravity (c.g).

To locate c.g. of regular objects:

The c.g of regular objects can be determined by balancing them on a knife edge or pivot. The c.g. of some
regular objects are described below will diagrams.

To determine position of c.m or c.g of plane lamina of irregular shape by plumb line
Apparatus: Irregular lamina, Stand, Plumb line, cork and pin.

1. Make three small holes near the edges of the lamina.

2. Suspend the lamina through one of the holes using a pin.
3. Hang a plumb line on the pin in front of the lamina.
4. When the plumb line is steady, draw a line on the lamina along the plumb line.
5. Repeat the above procedure for the remaining two holes.
6. The point of intersection of the three lines on the lamina is the position of centre of mass or centre of
gravity.

Precautions:

1. The lamina should be free to swing about its point of suspension.

2. The parallax error must be avoided

Stability of Objects:

“The ability of an object to regain its original position after it has been tilted slightly” determines the stability of
that object.

The stability of an object depends on:

a) Base are of object:

The area of the base of an object should be as wine as possible, for more stability.

e.g. the household objects like desk lamps have wide and heavy bases for more stability.

b) The position of c.m or c.g:

The centre of gravity (c.g) of an object should be as low as possible, for more stability.

e.g. the bus is not as stable as racing car, when they come to take a corner at high speed. For safety reasons the
e.g of a vehicle should therefore be as low as possible.