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Summary of Kinematics (AS)

AS and A Level Physics Imran Mirza ( MSC Physics, PGCC Scotland, UK )

1 Kinematics Describing Motion

Sub-topics:

Speed

Average Speed

Distance and Displacement

Speed and Velocity

Displacement-Time Graph

Combining displacement

Combining velocities

Key Points

Mechanics is the branch of physics that describes motion. Kinematics describes howobjects

move. This unit will focus on how objects move in straight lines (forward and backward).

Definition: Kinematics - the study of motion in a straight line w/o regard to the causes of motion,

i.e. forces

- it can be 2-D

-motion described in 3 ways:

1. sentences in everyday language

2. Mathematically, using equations

3. Graphically

Speed Rate of change of distance is called speed

Scalars represent quantities. How many slices of pizza did you eat? How many steps are in

front of the building? The answer to these questions is a scalar quantity.

Vectors represent a magnitude and a direction. I walked 14 paces North. The magnitude in

7/31/2019 Summary _01.pdf

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Summary of Kinematics (AS)

AS and A Level Physics Imran Mirza ( MSC Physics, PGCC Scotland, UK )

Distance andSpeedare scalar quantities. If I say, Travel 15 miles to get to my house, it may be

difficult to find, because there are no directions given. Do I mean straight line travel? Is 15 miles the

distance by road or by air?

Similarly, 50mph tells how fast an object is moving, but we dont know any more about the object. Is it

moving forward? Backward? We dont know.

timecedisspeed tan v = d/t

Speed is a rate at which distance is covered. Rate is a clue that something is divided by time.

Ex. Joan walks four blocks, turns around, and walks two blocks. She walks for 90 seconds.

How far did Joan walk?

How fast did Joan walk?

Displacementand Velocityare vector quantities.

Displacement how far from where we started. Velocity how fast we are moving in a direction

Speed - how fast something is moving.

Distance - a measurement of space between two objects.

Ex. Joan walks four blocks East, then turns around and walks two blocks West. Joan walks for three minutes.

What is Joans displacement from start to finish?

What is Joans average velocity?

time

ntdisplacemevelocityaverage_

Where v = velocity, d = displacement (in meters), t = time (in

seconds)

What are some appropriate units for velocity?

Ex: What is Joans average velocity if she walks an additional two blocks West? Why?

Which quantities give more information, scalars or vectors?

Ex. If a car travels 500 m North and 250 m South in 1 minute, what is its:

a) Distance traveled?b) Displacement from where it began?

c) Speed?

d) Velocity?

Ex. 2 If one car travels 100 m East for 5 seconds and another car travels 80m West for 4 seconds. What can

we say about the motion of the cars?

Average velocitydescribes the objects velocity over a period of time.

Instantaneous velocitydescribes the objects velocity at one instant in time.

With all that you now know, can you define constant velocity?

7/31/2019 Summary _01.pdf

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Summary of Kinematics (AS)

AS and A Level Physics Imran Mirza ( MSC Physics, PGCC Scotland, UK )

Distance/Displacement-time graphs (

POINTS TO BE NOTED

From X-axis we will find the time of journey

From Y-axis we will find the distance/displacement of the journey

Slop/Gradient of the graph will give us the speed/Velocity of the moving object

The simplest form of graph that describes the way an object is moving is a distance-time

graph. The distance that and object has moved is plotted on the Y-axis and the time is plotted

on the X-axis.

The graphs below show you how the distance changes with time for a number of different

examples.

Imagine that the distance axis shows how far an object is away from you.

The horizontal straight line shows something that stays at the same distance from you all the

time the object is stationary.

Straight lines slanting upwards show objects moving away from you at a steady speed while

straight lines slanting downwards show objects moving towards you at a steady speed. The

steeper the line the faster the object is moving.

A curved line shows an object whose speed is changing as time goes by.

We can use these graphs to work out the speed of an object.

This is easy to do for a straight-line graph but for the curved line the speed is constantly

changing and so we must measure the change in speed over a small time interval to get an

accurate answer.

Distance

Time

Moving away and getting faster

Moving away quickly

Stationary

Moving back

Moving away slowly

Speed = Distance/time and so the gradient of the line (distance/time) gives us the speed.