Acceleration (AQA GCSE Physics)

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Acceleration

Defining 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
  • The equation below is used to calculate the average acceleration of an object:

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

  • The change in velocity is found by the difference between the initial and final velocity, as written below:

change in velocity = final velocity − initial velocity

  • The equation for acceleration can be rearranged with the help of a formula triangle as shown:

Acceleration Formula Triangle, downloadable IGCSE & GCSE Physics revision notes

Acceleration, change in velocity, and time formula triangle

Speeding Up and Slowing Down

  • An object that speeds up is accelerating
  • An object that slows down is decelerating
  • The acceleration of an object can be positive or negative, depending on whether the object is speeding up or slowing down
    • If an object is speeding up, its acceleration is positive
    • If an object is slowing down, its acceleration is negative (sometimes called deceleration)

  • Two examples of accelerating and decelerating objects are shown in the image below

Acceleration Examples, downloadable IGCSE & GCSE Physics revision notes

A rocket speeding up (accelerating) and a car slowing down (decelerating)

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.

Part (a)

Step 1: List the known quantities

    • Initial velocity = 50 m/s
    • Final velocity = 42 m/s

Step 2: Write the relevant equation

change in velocity = final velocity − initial velocity

Step 3: Substitute values for final and initial velocity

change in velocity = 42 − 50 = −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

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

a = −8 ÷ 30 = −0.27 m/s

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.

Estimating Accelerations

  • The acceleration of an object is a measure of how quickly its velocity changes
  • A typical family car, for example, takes around 10 seconds to go from 0 m/s to 27 m/s (roughly 60 mph)
    • This is an acceleration of about 2.7 m/s2
    • The table below gives some other typical accelerations:

Typical Accelerations TableTypical Accelerations, downloadable IGCSE & GCSE Physics revision notes

Examiner Tip

You should be able to estimate the magnitude of everyday accelerations. Memorise the examples given in the table to develop a sense of the magnitude of different accelerating objects.

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.