Q1.
Take a thin metal strip of about 50 cm long. Hold its one end pressed to a table. You may use a few books or a brick to hold it (Fig. 5.17). Press the free end of the strip slightly and let it go. Observe the motion of this end of the strip. Does it move up and down?
Answer
Yes. The free end springs up and down about its original straight position, very fast at first and then more and more gently until it stops. This is also oscillatory motion.
The strip moves to and fro about a fixed position (its resting level).
∴ its motion is oscillatory motion.
∴ its motion is oscillatory motion.
Compare it with the hanging eraser of Activity 5.5:
| Hanging eraser | Metal strip | |
|---|---|---|
| Direction of the to-and-fro motion | Sideways (left–right) | Up and down |
| Fixed position it moves about | The lowest point of the arc | The level at which the strip rests |
| What brings it back each time | The pull of the Earth | The springiness of the metal |
| Type of motion | Oscillatory | Oscillatory |
Why up-and-down still counts: the definition says nothing about direction. Any repeated to-and-fro motion about a fixed position is oscillatory — sideways like a swing, up and down like this strip, or in and out like the skin of a drum.
Did you know? If you press the strip only a little and let it go, it oscillates so fast that you hear it as a low buzz. Every sound you hear is produced by something oscillating — a strip, a string, a drumhead, or your own vocal cords.