Round and round
Show on a big screenA potter's wheel turns at a steady speed. A lump of clay is on its rim, the outer edge. Does anything about the lump change?
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- A lump rides the rim. At the top its velocity points right.
- A quarter turn on, velocity points down. Same length.
- Halfway round, velocity points left. Same length.
- Three-quarters round, velocity points up. Same length.
Yes. The speed stays the same, but the velocity's direction keeps changing.
The rule
In uniform circular motion, the speed is constant. The direction of the velocity keeps changing. So the motion is accelerated.
A line that touches a circle at one point is a tangent. The velocity at each point points along the tangent.
A marble goes round inside a ring on a smooth table. Take the ring away. The marble then moves in a straight line, along the tangent.
One lap is once all the way round. The radius is the distance from the centre to the rim. The distance round a circle is 2 × π × radius. Here we take π = 22/7.
| Quantity | Working | Answer |
|---|---|---|
| Distance | 2 × 22/7 × 7 | 44 m |
| Displacement | Ends where it began | 0 m |
| Average speed | 44 ÷ 22 | 2 m/s |
| Average velocity | 0 ÷ 22 | 0 m/s |
Try the lab to finish this lesson.