Fast does not mean accelerating
Acceleration depends on how quickly velocity changes.
A smooth highway
A bus moves along a long straight highway at a very high, steady velocity. The passengers feel no jolt.
Do you think the bus has a large acceleration because it moves so fast?
What this lesson covers
The idea
Acceleration depends not on how fast an object moves but on how quickly its velocity changes, so an object moving very fast at constant velocity has zero acceleration.
A smooth highway
A bus moves along a long straight highway at a very high, steady velocity. The passengers feel no jolt.
Do you think the bus has a large acceleration because it moves so fast?
- No, its velocity is not changing
- Yes, because fast things have large accelerations
- Yes, because the highway is long
Velocity or change in velocity?
Set the two sliders as asked and watch the acceleration.
Not how fast, but how quickly it changes
At 30 m s⁻¹ with no change in velocity the acceleration was zero. At only 5 m s⁻¹ with a change of 10 m s⁻¹ in 5 s it was 2 m s⁻². The acceleration followed the change in velocity, not the velocity itself.
Acceleration depends not on how fast an object is moving, but on how quickly its velocity is changing. An object moving very fast at constant velocity has zero acceleration.
Notes
Acceleration depends on how quickly velocity changes, not on how fast the object moves. Constant velocity means zero acceleration.
Check yourself
A bus moves along a straight highway at a constant velocity of 25 m s⁻¹. Its acceleration is…
Acceleration depends on…
The velocity of a vehicle changes from 5 m s⁻¹ to 15 m s⁻¹ in 5 s. What is its average acceleration?
Answer: 2 m s⁻²
(15 − 5) m s⁻¹ ÷ 5 s = 2 m s⁻².
Car P moves at a constant 30 m s⁻¹. In 5 s the velocity of car Q goes from 0 to 10 m s⁻¹. Which has the larger acceleration?
- 25 m s⁻². 25 m s⁻¹ is the velocity. Acceleration is the change in velocity per second, and here the velocity does not change.
- very large, because it is moving fast. Acceleration does not depend on how fast the bus moves. It depends on how quickly the velocity changes.
- zero — correct. Yes!
- how quickly the velocity is changing — correct. Yes!
- how fast the object is moving. A fast object at constant velocity has zero acceleration, so the speed alone does not decide it.
- how far the object has moved. The distance moved does not decide the acceleration. The change in velocity in a time interval does.
- Car P. Car P has zero acceleration because its velocity does not change.
- Car Q — correct. Yes!
- Both have the same acceleration. Car P has zero acceleration, but car Q has 2 m s⁻².