1.
Motion, Forces and Energy
1.1Physical Quantities and Measurement techniques
Measuring lengths: -
- We use rulers to measure lengths
- Measuring cylinders to measure volume
- Micrometre screw gauges to measure very small distances accurately
Scalar: -
- Quantities that only have magnitude
- Distance is also scalar as it has no direction
- E.g. Speed, time, mass, energy and temperature
Vectors: -
- Quantities that have both magnitude and direction
- Velocity is also vector because it is necessary to mention both its speed
aandd the direction
- E.g. Force, weight, acceleration, momentum, electric field strength and
the gravitational field strength
Resultant of vectors: -
There are two ways to calculate: -
1. Calculating
- Calculate using Pythagoras’ theorem a 2+ b2=c 2
2. Graphically
1.2 Motion
Speed and Velocity: -
- Speed is the distance travelled per unit time, speed does not have
direction and so is a scalar quantity
- Velocity is almost the same as speed, except for one major difference,
velocity has a direction and so is a vector quantity
- Velocity is the distance you travel in a specific amount of time in a
specific direction
To calculate velocity, you use: -
1. Motion, Forces and Energy
distance travelled
velocity=
timetaken
To calculate average speed, you use: -
total distance travelled
average speed=
total timetaken
Acceleration: -
- Acceleration is the rate of change of velocity
To calculate acceleration, we use: -
v
a=
t
Speed time Graphs: -
- A speed time graph shows how an objects speed changes over time
using a line or curve in a graph
e.g. this table shows the speed of a n object at intervals of 1 second
Time (s) Speed (m/s)
0 10
1 10
2 10
3 10
4 10
- As you can see, the speed of the object does not change during the 5
seconds, we can say this object has a constant speed which means the
graph will be a straight line
Calculating the distance from speed-time graph: -
- Calculate the are under the graph
1. Motion, Forces and Energy
- First, we divide the shape, then we find the areas
- For the triangle t is 1 x 4 x half which is 2.
- Then we calculate the area of the rectangle which we can find by doing –
4 x 4 as when we are doing subtraction from 5 and 1 in time and taking 4
from the speed
- Then finally we get the answer as 18 (2 + 16)
- Deceleration is negative acceleration (if deceleration is 2, acceleration
will be -2)
Calculating speed from a distance time graph: -
- U can simply find the gradient or slope
y 2− y 1
- x 2−x 1
- For this, you have to take any to points on the line say, (1,2) and (2,6)
- Then you have to substitute, in y2 you can right 6 and y1 u can write 2,
and following that u will get the answer
The three equations of motion: -
Here are some equations to solve sums easily: -
First learn the symbols: -
v = Final Velocity
u = Initial Velocity
1. Motion, Forces and Energy
a = Acceleration
t = Time
s = Distance
g = Gravity (acceleration of mass 9.8 metre per second square)
Equations: -
1) v=u+at
1 2
2) s=ut+ 2 a t
3) v 2−u2=2 as
Free Fall: -
- When an object falls freely under the influence of gravity alone, without
any other forces affecting its motion.
- Air resistance is a force that slows down objects falling through the air
- When an object is in free fall, its acceleration will be 9.8m/s 2
- Without air/liquid resistance, falling objects accelerate continuously.
- With resistance, acceleration decreases and they eventually reach a
constant speed known as terminal velocity.
- The acceleration of a falling body does not depend on the mass of the
body (be the body be heavy or light, when falling, it will touch the
ground at the same time).
2.2Mass and Weight
- Mass and weight are completely different.
- Mass is a measure of the amount of matter in an object when it’s not
moving.
- Weight is a gravitational force on an object that has mass.
- Weight is the effect of a gravitational field on a mass.
- Strength of gravitational field can be described as force per unit mass
- To calculate the gravitational field strength (g)
w
g=
m
2.3Density
1. Motion, Forces and Energy
- If the density of an object is > that of liquid it will sink, but if the density
of the object is < that of liquid it will float.
-
Density: mass per unit volume of a substance
Mass: measurement of how much stuff is in an object
Volume: the amount of space that it occupies or fills
Density can be calculated from: -
mass
density =
volume
Regularly shaped solid: -
- Mass – top pan balance
- volume - measuring its dimensions with a ruler.
Irregularly shaped solid: -
- The mass - a top pan balance
- The volume - displacement methods
Method 1: -
- take a beaker fill it with water
- Note the volume (initial volume)
- Slowly put the object inside
- Note the volume (final volume)
- Subtract (final – initial volume)
- 7
1. Motion, Forces and Energy
Method 2: -
Liquid: -
- Measure the mass of an empty beaker (m1)
- Pour water inside (m2)
- M2 – M1
Air: -
- Measure the mass of a flask full of air (m1)
- Remove the air through the vacuum (m2)
- subtract
- The volume of air is found by filling the flask with water and pouring it
into a measuring cylinder.
Floating and sinking: -
1. Motion, Forces and Energy
- An object sinks in a liquid of lower density than its own otherwise it
floats, partly or wholly submerged.
- A liquid of low density will float on a liquid of higher density if the two
liquids do not mix.
Q. Describe how you could determine density of a liquid.
- Measure the mass of an empty measuring cylinder. (m1).
- Fill the measuring cylinder with liquid. (m2).
- Mass of liquid = m2 – m1
- Density of a liquid = m2 – m1 divide by volume
