I'm studying for my upcoming AP Physics 1 exam but can't figure out this problem
A student shakes a horizontally-stretched cord, creating waves. The graph above shows the vertical position $y$ as a function of time $t$ for a point on the cord. The student then tightens the cord so that waves on it will travel faster than before. How should the student now shake the cord to make the graph of $y$ versus $t$ for the point look the same as above?
(A) With fewer shakes per second than before
(B) With the same number of shakes per second as before
(C) With more shakes per second than before
(D) The answer cannot be determined without knowing the wavelength of the waves.
My intuition would tell me that increasing the speed of the waves would cause the point to oscillate at a faster rate vertically, thus fewer shakes per second than before are needed to maintain the same frequency for the particle's oscillation. However, the correct answer is B, so I really need a thorough explanation as to why the answer is B.