AP Physics 1 - Harmonic Motion

Question

Question

A long spring runs across the floor. A pulse is sent along the spring. After a few seconds, an inverted pulse returns. Is the spring attached to the wall or lying loose on the floor? Why?

Answer: Attached to a wall. When a wave hits a fixed endpoint (eg attached to the wall), the wave pulls on the wall as it hits the end point, but at the same time, the wall pulls on the wave in the opposite direction with equal force. This produces a wave traveling back the opposite direction but inverted. I know the answer, but I need an explanation in laymen's terms.

Answer 1

It might help to think of the pulse as a push. So, the pulse travels all the way across the spring and then tries to push the wall. But the wall is stationary. So, it simply pushes back with an equal force (Newton's 3rd law), hence generating the same pulse but in the opposite direction.

Answer 2

You could imagine an analog situation. Your spring is twice as long as in the case with the wall. What could you do to ensure that the spring doesn't move in the middle (where the wall was) is to ask a friend to send a pulse from the other end of the spring at the same moment as you. If his pulse has the same shape as yours but in the opposite (transverse) direction, the will interfere destructively precisely in the middle of the spring.

If your friend now moves toward the middle of the spring you could repeat the experiment but this time he would have to send his pulse later since he is closer to the middle of the spring (in order to cancel the mouvement of the spring in the middle). What happen if he moves to the middle of the spring ? Well, he would have to send his pulse exactly when he receives yours and you can convince yourself that even though you'll receive his pulse, his hand would not move while he is cancelling your pulse. The situation is now similar than the one with the wall (your friend is now the wall).