Acceleration (Edexcel GCSE Physics)
Revision Note
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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
Δv = v − u
Where:
v = final velocity in metres per second (m/s)
u = initial velocity in metres per second (m/s)
The equation for acceleration can be rearranged with the help of a formula triangle as shown:
Speeding Up & 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)
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/s2In other words, acceleration measures how much the velocity (in m/s) changes every second, m/s/s.
Freefall
In the absence of air resistance, all objects fall with the same acceleration
This is called the acceleration due to gravity:
g = acceleration due to gravity = 10 m/s2
In the absence of air resistance, Galileo discovered that all objects (near Earth's surface) fall with an acceleration of about 10 m/s2
This means that for every second an object falls, its velocity will increase by 10 m/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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