Section 1 of 5Acceleration Ideas
Acceleration describes how quickly the velocity of an object is changing.
Definition
- Definition Acceleration is the rate of change of velocity, so it tells you how quickly the velocity of an object changes.
Vector
- Vector Acceleration is a vector, so its direction matters, and a change of direction is also an acceleration even when the speed is constant.
Units
- Units Acceleration is measured in metres per second squared (m/s²), because it is a change in velocity in m/s divided by a time in s.
Deceleration
- Deceleration Deceleration is an acceleration opposite to the direction of motion, which causes an object to slow down.
Section 2 of 5Speeding Up and Slowing Down
The direction of the acceleration compared with the motion decides whether an object speeds up or slows down.
Speeding Up and Slowing Down
Speeding up
- Speeding up Velocity increases, and the acceleration acts in the same direction as the motion.
Slowing down
- Slowing down Velocity decreases, and the acceleration acts opposite to the direction of motion.
Constant velocity
- Constant velocity There is no change in velocity, so the acceleration is zero.
Turning
- Turning The direction changes while the speed stays constant, so there is an acceleration towards the centre of the turn.
| Situation | Change in velocity | Acceleration |
|---|---|---|
| Speeding up in a straight line | Increases | In the direction of motion |
| Slowing down in a straight line | Decreases | Opposite to the direction of motion |
| Moving at constant velocity | No change | Zero |
Section 3 of 5Using the Formula
Use the same steps every time to avoid sign and unit errors.
1
Write down the initial velocity u, the final velocity v and the time taken t.
2
Find the change in velocity by subtracting: v − u.
3
Divide the change in velocity by the time: a = (v − u) ÷ t.
4
Check that the unit is m/s² and that the sign shows whether the object is speeding up or slowing down.
Section 4 of 5Worked Example
Question
- A car increases its velocity from 4 m/s to 20 m/s in 8 s.
- Calculate its acceleration.
- Change in velocity = final velocity − initial velocity = 20 − 4 = 16 m/s.
- Acceleration = change in velocity ÷ time = 16 ÷ 8.
- Acceleration = 2 m/s².
Answer The acceleration of the car is 2 m/s².
Section 5 of 5Exam Checklist & Tips
5.6.10 – Key Points for the Exam
- Acceleration The rate of change of velocity, measured in m/s².
- Velocity Speed in a stated direction, measured in m/s.
- Deceleration A decrease in speed, which is an acceleration opposite to the direction of motion.
- Uniform acceleration Acceleration that stays constant over a period of time.
- Initial velocity The velocity of an object at the start of the time period being considered.
Common Mistakes
- Calling acceleration just speed; an object moving at constant speed around a bend is still accelerating.
- Forgetting to subtract the initial velocity, so using v ÷ t instead of (v − u) ÷ t.
- Writing the unit of acceleration as m/s instead of m/s².