# Speed of a wave problem

The figure shows a snapshot graph of a wave traveling to the right along a string at 45 m/s. Find velocity at 1, 2, and 3

I don't know how to approach this problem since I thought the speed of a wave traveled the same speed on all of the string but it tells me that 45m/s is wrong.

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vanesch
Staff Emeritus
Gold Member
I don't know how to approach this problem since I thought the speed of a wave traveled the same speed on all of the string but it tells me that 45m/s is wrong.

I think they are asking for the vertical velocity of the matter point on the string... You have all the elements: you know the phase velocity and wavelength of the wave, and from the figure you can also read its amplitude (and phase). Write down the mathematical expression for the wave (hint: will be something like sin or cos of something), and then look at the motion of a point at fixed value of x...

they have a equation that says Vy=-omega*Acos(kx-omega*t+phase constant)
I dont know what t or x stand for so how would I calculate this and isn't A at the point 0 equal to 0 therefore the velocity should be 0. I dont get how this equation is used and I cant find an example of it.

vanesch
Staff Emeritus
Gold Member
I dont know what t or x stand for t stands for time, and x stands for the x-coordinate (along the wave motion) of course !

I know what they stand for but I dont know what they are. How do I find these values? Also what do I do about A since it should be 0 at the point 0 so shouldn't the answer be 0. Everytime I try this equation I get the wrong answer.

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Doc Al
Mentor
I know what they stand for but I dont know what they are. How do I find these values?
Since you have the speed of the wave and the wavelength, you should be able to figure out the frequency.

Also what do I do about A since it should be 0 at the point 0 so shouldn't the answer be 0.
A is the amplitude of the wave--it's not zero.
Everytime I try this equation I get the wrong answer.
You might need to review what the equation means; try reading http://hyperphysics.phy-astr.gsu.edu/hbase/waves/wavsol.html#c4"

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Velocity of Points

I am trying to create the equation for the line like you said, i have solved values for A, f, w, and k. However, I cannot find the phase constant. Also, once I do have the equation, how it help me find the velocity at the different points?