Teaching Physics with the Physics Suite Edward F. Redish

Motion in a standing wave

A string is connected at one end to a vibrating reed and on the other to a weight pulling the string tight over a pulley. At the time t = 0 a photograph of the string looks like the picture at the left below. The wave is then observed to grow in amplitude, reaching a maximum of 3 cm at t = 0.02 sec. It then shrinks in amplitude, and then at time t = 0.06 sec appears flat, as shown in the picture to the right below.

Each box in the grid has a side of 1 cm.

1. Two points on the string are marked with heavy black dots and with the letters A and B. At t = 0, in what direction is each of them moving and which one (if any) is moving faster?
2. At t = 00.06, mark the position of the two points on the string on the figure at the right. In what direction is each of them moving and which one (if any) is moving faster?
3. What is the period of the oscillation?
4. Explain what could be changed to make the velocity of point A zero throughout the oscillation. Give two different ways to do this, and explain why such a change would work.

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