Acceleration (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Written by: Leander Oates, Physics

Reviewed by: Caroline Carroll, Physics Subject Lead

Acceleration

What is acceleration?

Acceleration is defined as the rate of change of velocity
In other words, it describes how much an object's velocity changes every second
Acceleration is a vector quantity, it has both magnitude and direction
Any change in velocity is acceleration
This includes:
- Speeding up
- Slowing down
- Changing direction

As it is a vector quantity, the sign of acceleration tells us its direction, depending on which direction is defined as positive
- If an object is speeding up (accelerating), its acceleration has the same sign as its velocity
- If an object is slowing down (decelerating), its acceleration has the opposite sign to its velocity

Acceleration & Deceleration

An accelerating rocket and a decelerating car, for IGCSE & GCSE Physics revision notes — **A rocket speeding up (accelerating) and a car slowing down (decelerating)**

Calculating average acceleration

Average acceleration can be calculated using:

$a = \frac{∆ v}{t}$

Where:
- a = average acceleration measured in metres per second squared (m/s²)
- Δv = change in velocity measured in metres per second
- t = time measured in seconds (s)

Change in velocity can be found using:

$∆ v = v_{f} - v_{i}$

Where:
- Δv = change in velocity
- v_f = final velocity
- v_i= initial velocity

Worked Example

A Japanese bullet train decelerates at a constant rate in a straight line. The velocity of the train decreases from 50 m/s to 42 m/s in 30 seconds.

(a) Calculate the change in velocity of the train.

(b) Calculate the deceleration of the train, and explain how your answer shows the train is slowing down.

Answer:

Part (a)

Step 1: List the known quantities

Initial velocity, v_i = 50 m / s
Final velocity, v_f = 42 m / s

Step 2: Write the relevant equation

$∆ v = v_{f} - v_{i}$

Step 3: Substitute values for final and initial velocity

$∆ v = 42 - 50$

$∆ v = - 8 m / s$

Part (b)

Step 1: List the known quantities

Change in velocity, Δv = − 8 m/s
Time taken, t = 30 s

Step 2: Write the relevant equation

$a = \frac{∆ v}{t}$

Step 3: Substitute the values for change in velocity and time

$a = \frac{- 8}{30}$

$a = - 0.27 m / s^{2}$

Step 4: Interpret the value for deceleration

The answer is negative, which indicates the train is slowing down

Examiner Tip

Remember the units for acceleration are metres per second squared, m/s². In other words, acceleration measures how much the velocity (in m / s) changes every second, m/s/s.

Last updated: 13 April 2024

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