Acceleration (AQA GCSE Physics)
Revision Note
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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, 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
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.
Answer:
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 Tips and Tricks
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 Table
Examiner Tips and Tricks
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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