1. The problem statement, all variables and given/known data Suppose you're operating a machine with two motors. If the two motors are not operating at the same rpm, you will hear a periodic variation in the intensity of the sound-that is, beats. Suppose the tachometer that indicates the rotation rate of one motor is malfunctioning. The operational one shows 3600 rpm. You hear a beat every 21 s. You are certain the other motor is running fast. What is its rotational speed in rpm? 2. Relevant equations w1-w2 is the frequency of amplitude change (according to the textbook). 3. Attempt at solution delta_w = 2pi/T = w1-w2 w1 - 2pi/T = w2 w2 is not valid (given that I have converted all units to standard and back into rpm). So I think my formula isn't valid. Can someone derive a correct one please? Thanks.