You are videotaping the car in the photo below at a rate of

You are videotaping the car in the photo below at a rate of 30 frames per second (fps), a) At what rotation rate (in RPM) will the wheel appear rotating backwards in the video? b) At what rotation rate will the wheel appear stopped in the video?

Solution

Recording speed 30 frames per second.

When the speed of the wheel matches the speed of recording i.e 30 revolutions per second (30x60 = 1800rpm), the wheel appears to be stationary.

When the speed is more tham 30 revolutions per second (1800 rpm), the wheels appear to rotate backwards.