Teaching Physics with the Physics Suite Edward F. Redish

Parsing a pulse

 An instructor is demonstrating the motion of waves on a long, taut spring. He is holding the spring at one end and will move it so the spring will move back and forth on the floor. The spring is rigidly connected to a metal rod at its other end. The spring is under a tension T and it has a mass density m. The instructor starts a pulse moving towards the right as shown in the figure below. The pulse is triangular and is not symmetric. The figure is shown at a time t0.

1. Calculate the time t it will take the peak of the pulse to reach the wall (to travel a distance s).

2. What will the spring look like at the time t0 + t? Draw a carefully constructed and labeled diagram to show what it looks like and how you got your result.

3. What will the spring look like a bit later ­ say at a time t0 + 2t? What is responsible for this result?
4. The width of the pulse is 0.5 m. If the tension in the spring is 5 N and it has a mass density of 0.1 kg/m, how much time did the professor take to generate the pulse?

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