A string is 37.5 cm long and has a mass per unit length of 5.95 10-4 kg/m. What tension must be applied to the string so that it vibrates at the fundamental frequency of 612 Hz?

Question

The answer is supposed to be 125 N, but I'm not sure how to get this answer. So far, I have found the mass of the string to be 2.0235 x 10e-04 kg. I'm not sure if this is useful or not, though. Any guidance is greatly appreciated.

Answer 1

You need to know that the formula for the speed of travelling waves on a taut string is

V = sqrt(T/d).

where d is the lineal density per unit length, and T is the tension.

The fundamental frequency has a wavelength of twice the length, or 0.75 m. A standing wave at the fundamental frequency is two traveling waves going in opposite drections with wavelength 0.75 m and wave speed
V = (wavelength)*(frequency)
= (0.75 m)*612 Hz
= 459 m/s

Now you can use the wave speed equation to solve for the string tension, T

T = d*V^2 = 5.95*10^-4 * 459^2 = 125 N