Two ways to change velocity
Acceleration from a change in magnitude, in direction, or both.
A bend in the road
A car goes round a bend. The needle of its speedometer does not move at all.
Do you think the velocity of the car is changing?
What this lesson covers
The idea
Average acceleration can result from a change in the magnitude of velocity, a change in its direction, or both.
A bend in the road
A car goes round a bend. The needle of its speedometer does not move at all.
Do you think the velocity of the car is changing?
- Yes, its direction is changing
- No, because its speed is not changing
- It cannot be said
Three cases
Open each tab. Each red arrow shows the velocity of a car at that point.
What can change
Velocity has a magnitude and a direction, so either of them can change. In the first tab only the magnitude changed, in the second only the direction, in the third both. In every case the velocity changed, so there was an average acceleration.
Average acceleration can result from a change in the magnitude of velocity, a change in its direction, or both.
Notes
Acceleration can come from a change in the magnitude of velocity, in its direction, or both.
Check yourself
A car goes round a bend with the speedometer needle steady. Can it have an average acceleration?
Average acceleration can result from…
A bus on a straight highway speeds up from 10 m s⁻¹ to 15 m s⁻¹ in the same direction. Does it have an average acceleration?
- No, because its speed is not changing. The velocity has a direction too. When the direction changes, the velocity changes and there is an average acceleration.
- No, because the road is not straight. The road being curved is not the reason. What matters is that the velocity changes, and here its direction does.
- Yes, because the direction of its velocity changes — correct. Yes!
- a change in the magnitude of velocity, a change in its direction, or both — correct. Yes!
- only a change in the magnitude of velocity. A change in the direction of the velocity can also give an average acceleration.
- only a change in the direction of velocity. A change in the magnitude of the velocity can also give an average acceleration.
- No, because its direction does not change. A change in the magnitude of the velocity alone is enough for an average acceleration.
- Yes, because the magnitude of its velocity changes — correct. Yes!
- No, because it is not going round a bend. Acceleration does not need a bend. A change in the magnitude of the velocity is enough.