Steady and changing motion
Show on a big screenTwo trolleys start together. One keeps a steady speed. One gains speed every second. Which is ahead?
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- Time runs along the bottom, distance up the side.
- Trolley A holds a steady 4 m/s: a straight line.
- Trolley B starts at rest and gains 2 m/s each second: a curve.
- Both reach 16 m at t = 4 s.
Trolley A is ahead until t = 4 s. Then both are level at 16 m. After that, Trolley B is ahead.
The rule
Uniform motion covers equal distances in equal times, however short. Otherwise the motion is non-uniform.
In uniform motion the speed is constant: it does not change. In non-uniform motion the speed rises, falls, or both.
Successive seconds are one second after another. If the distances in successive seconds grow, the speed is increasing. If they shrink, the speed is decreasing.
| Second | 1st | 2nd | 3rd | 4th | 5th |
|---|---|---|---|---|---|
| Trolley A, steady | 4 m | 4 m | 4 m | 4 m | 4 m |
| Trolley B, speeding up | 1 m | 3 m | 5 m | 7 m | 9 m |
Each second, Trolley B covers 2 m more than before. So its velocity gains 2 m/s every second. That is constant acceleration: the same gain each second.
Try the lab to finish this lesson.