Motion in a Straight Line

Physics: AQA A Level, topic 4.1.7

Key Concepts of Motion

Displacement, speed, velocity and acceleration are fundamental concepts when describing motion in a straight line. You need to know how they link together.

Displacement

The displacement of an object, s, is the distance and direction from an object's starting position to its final position.

Velocity and speed

The velocity of an object, v, is the speed and direction an object is travelling.
It is the rate of change of displacement:
$v = \frac{\Delta s}{\Delta t}$
Speed is the magnitude of the velocity.
Instantaneous speed/velocity is the speed/velocity of an object at a given instant in time.
Average speed/velocity is the total distance/displacement divided by the total time taken.

Acceleration

The acceleration of an object, a, is the rate (and direction) at which its velocity changes:
$a = \frac{\Delta v}{\Delta t}$

Delta symbol

Delta, Δ, is the symbol meaning "change in".
For example, Δs means change in displacement.

Uniform Acceleration Equations

These four equations describe motion along a straight line with uniform (constant) acceleration or deceleration.

Using the equations

You will often be asked questions involving the 5 key concepts of motion in a straight line:
s, displacement
u, initial velocity
v, final velocity
a, acceleration
t, time
Usually you will be given three and asked to calculate a fourth using the 'suvat' equations, which are as follows...

The equations

$v = u + at$
$s =(\frac{u + v}{2})t$
$s = ut + \frac12 at^2$
$v^2 = u^2 + 2as$

Test yourself on Motion in a Straight Line

What does s represent?
What is the correct formula for acceleration?
Which equation for uniform displacement is correct?
Which suvat equation does NOT contain displacement?

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