Now set the tension to low and wiggle the wrench to create more waves. Can you explain how moving the first point on the string, the one closest to the wrench, affects the next point on the spring? How does this fit with your understanding of the force of a stretched spring?

When the string moves, it creates a very small change in the distance to the next point, th

Explanation:

When the string moves, it creates a very small change in the distance to the next point, this generates a restoring force that tends to push the string back, this small disturbance propagates along the string and is what creates the pulse.

This is similar to what happens when a spring is stretched and a restoring force is generated shaved by the law of shortening.

            F = k Dx

As the first particle travels upward, it pulls on the next particle, which follows the first particle upward. Then next in line is pulled up in turn, and so on. The motion of each particle follows the one before it, either up or down, with a slight lag in time. This succession of particles moving up or down travels along the string as a wave. Eventually, it pulls every particle along the string up and down in series.

Explanation: