Q1.
Tie an eraser (or a potato) to one end of a thread. Hang the eraser by holding the other end of the thread (Fig. 5.16). Keep your hand steady. Using the other hand, take the eraser slightly to one side and then release. Does it start moving to and fro? Is its motion similar to the motion of a swing?
Answer
Yes to both. The eraser swings to and fro about the lowest position, and its motion is exactly like a swing in a park. This is oscillatory motion.
When an object moves to and fro about some fixed position,
its motion is called oscillatory motion.
its motion is called oscillatory motion.
What you will see:
- The eraser passes through the same lowest point every time — that is the fixed position it oscillates about.
- It repeats its path over and over: right, lowest point, left, lowest point, right…
- Each complete to-and-fro trip takes the same time, so the motion is also periodic.
- The swings become smaller and smaller and finally stop, because air resistance and friction at the top slowly take away its motion.
| Hanging eraser | Swing in a park | |
|---|---|---|
| Fixed point at the top | Your steady hand | The bar of the swing frame |
| What moves | The eraser at the end of the thread | The seat with the child on it |
| Path | A short arc, repeated to and fro | A short arc, repeated to and fro |
Why your hand must be steady: the oscillation is measured about a fixed position. If your hand wanders, the lowest point wanders too, and you no longer have a clean to-and-fro motion to observe. The same is true of a wall-clock pendulum — the clock case holds the top absolutely still.
Try This: Count how many complete to-and-fro trips the eraser makes in 30 seconds with a 30 cm thread, and then with a 60 cm thread. The longer thread gives fewer trips — the very fact that makes a pendulum useful for keeping time.