Q1.
Tie an eraser (or a potato) to one end of a thread. Hold the other end of the thread with your hand and whirl it (Fig. 5.15). Observe its motion. Is the motion of the eraser the same as that of a merry-go-round?
Answer
Yes — the two are the same kind of motion. The eraser goes round and round along a circular path, and so does a child sitting on a merry-go-round. Both are examples of circular motion.
When an object moves along a circular path,
its motion is called circular motion.
its motion is called circular motion.
| Whirling eraser | Merry-go-round | |
|---|---|---|
| Centre of the circle | Your hand | The central pole |
| What keeps it on the circle | The thread pulls it inward | The metal arms pull the seat inward |
| Distance from the centre | Always the same (the length of the thread) | Always the same (the length of the arm) |
| Path traced | A circle, repeated again and again | A circle, repeated again and again |
The key test for circular motion: the moving object stays at a constant distance from one fixed point. Measure the thread — say 40 cm — and the eraser is 40 cm from your hand at every instant of the whirl. That is exactly what makes the path a circle.
Check it yourself: While whirling, let go of the thread (do it outdoors, away from people and windows). The eraser flies off in a straight line, not in a circle — proof that it was the pull of the thread that was bending its path into a circle all along.