# Waves on a string.

1. Homework Statement
A series of pulses, each with an amplitude of .150 m are sent down a string that is attached to a post at one end. The pulses are reflected back and travel back along the string with no loss of amplitude. What is the net displacement of an element at a point on the string where the 2 pulses cross a. if the string is rigidly attached, b. if the end at which reflection occurs is free to slide up and down.

2. Homework Equations
I know that in a rigidly fixed string, reflection inverts the waves, and if the string is free to move up and down, the reflected pulse is not inverted.

3. The Attempt at a Solution
I am thinking that in a rigid system, the net displacement where the two pulses cross would be zero, because that point would be a node. If the system is free to move, then I think that the net displacement at the point where the waves meet would be plus or minus A. Is my brain working right?

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Tom Mattson
Staff Emeritus
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3. The Attempt at a Solution
I am thinking that in a rigid system, the net displacement where the two pulses cross would be zero, because that point would be a node.
Yes, the incoming and outgoing pulses are negatives of each other. Since waves add linearly, their sum is zero.

If the system is free to move, then I think that the net displacement at the point where the waves meet would be plus or minus A. Is my brain working right?
Hold up. Now the displacements of the incoming and outgoing waves have the same sign. How do numbers of the same sign add up?

So in the second one, the displacement would be 2A, because the sum of the 2 diplacements is (in this case) .300?

Tom Mattson
Staff Emeritus