Topic Seven (1.7): Kinematics Equations

Introduction

Kinematics studies how things move as time proceeds. It uses the five ‘suvat’ equations relating displacement, initial and final velocity, constant acceleration and time: These arise very simply from the shape of the speed-time graph (Topic 1.3). They are extremely useful because if you know any three quantities and want a fourth, you just pick the equation that involves those four values and rearrange it to find the answer.

The suvat equations are used to calculate different aspects of motion of an object. You will recognise $LaTeX: s=\frac{1}{2}gt^2$ $s=\frac{1}{2}gt^2$ , used to calculate the acceleration due to gravity. Note that LaTeX: g = a $g = a$ in this version.

The best way to tackle these questions is to write suvat down the side of the page and write down each piece of information you are given. Circle the one you want to calculate and cross out the one that is not needed. This will make it easier to identify which equation you need from the data book.

Reading

Edexcel International AS/A Level Physics Student Book 1 pages 23 to 25

Edexcel International A Level Physics Lab Book pages 6 to 10 (also discussed in Topic 1.3) and 71 to 77

Tasks

Complete the following:

Read pages 23 to 25 of the textbook.

READ – Motion Equations from the Velocity-time Graph Download READ – Motion Equations from the Velocity-time Graph To understand how these equations simply arise from the velocity-time graph.

WATCH – Examples of Using ‘Suvat’ Equations of Motion (Chris Gozzard) Links to an external site.

WATCH – Laws of Motion Examples (Helen Rogerson) Links to an external site.

COMPLETE – Gameboard: Equations of Motion – 1D (Isaac Physics) Links to an external site.

COMPLETE – Equations of Motion – Constant Velocity (Isaac Physics) Links to an external site.

COMPLETE – Equations of Motion 1D – Non-zero Acceleration (Isaac Physics) Links to an external site.

COMPLETE – Equations of Motion – Free Fall (1D) (Isaac Physics) Links to an external site.

Memorise the four ‘suvat’ equations in Table A at the bottom of page 24.
Complete the questions on page 25. You will find the answers in General Resources under the heading Textbook Answers. Download Textbook Answers.

Practical Knowledge

Determine the acceleration of a freely falling object. (Core Practical 1)
Read the Practical Skills box on page 25.

WATCH – ‘Determination of By Freefall Method’ (Malmesbury Education) Links to an external site.

Now try to write your own diagram and description of how to conduct this experiment safely and analyse the results.
Work through pages 6 to 10 in the Lab Book (see Topic 1.3).

Top Tips

Make sure that you understand the following key points:

List the five quantities $LaTeX: s\ u\ v\ a\ t$ $s\ u\ v\ a\ t$ , which three do you know, which one do you want to know.
Pick the equation that uses just those four quantities.
Avoid confusion: Be clear which direction you choose as positive. Keep to this consistently throughout the calculation!
Rearrange equations in symbol form, then enter all values (in the same format) into your calculator. Check all values are correctly entered, then you will always get the right answer!
‘Free-fall’ or ‘under gravity’ means $LaTeX: a\ =\ 9.81\ m\ s^{-2}$ $a\ =\ 9.81\ m\ s^{-2}$ .
Starting (or ending) ‘at rest’ means $u$ (or $v$ ) is zero.
The kinematic equations (suvat) are only valid if there is constant (‘uniform’) acceleration (e.g. no losses such as air resistance). If the acceleration is changing, they cannot be used.

Key Terms

Add the following key terms with definitions to your word list:

Kinematic
Displacement
Initial velocity
Final velocity
Acceleration
Time
Uniform motion
Uniform acceleration
At rest
Free-fall