# An unexpected error with waves

On the first waves exam I included a question where I gave students y vs. x plots of a wave at t=0 and at t=0.1 s. I then asked the following questions:

a) Determine the wavelength, period, and wave velocity.

b) Write the wave function y(x, t) that describes this wave.

Students are reading values off a graph so there is some wiggle room in the values they obtain for each quantity. Without thinking about it, I used a wave with an amplitude of 2.4 m and a wave speed of 2 m/s. While grading the exams, I came across several students with amplitudes in y(x, t) that were slightly off, but I chalked this up to being in a hurry and not reading the graphs carefully.

Then I came across a student who explicitly labeled the amplitude of y(x, t) as the wave speed. This error never occurred to me and because I chose such similar values for amplitude and wave speed I can’t determine if other students are making this error or if they are just sloppy graph readers. Sigh… something to be more careful about next time I write questions.