On television, rotating objects such as wagon wheels and airplane propellers often appear to be moving very slowly or even backwards. This is
a result of aliasing, caused by the sampling rate of the video (30 frames per second) being less than twice the frequency of the rotational motion. To understand this, imagine we paint one of the blades of an airplane propeller so that we can identify it from the other blades. We will then turn the propeller at 33 rotations per second, in a clockwise direction. In frame number 1 of our video sequence, the marked blade happens to be exactly at the top of the propeller.
a. How many rotations does the marked blade make between two successive frames?
b. Draw a sketch of how the propeller would appear in frames 1, 2, 3 and 4.
c. How many frames does it take for the marked blade to again appear at the top?
d. What rotational frequency is (c) in rotations per second?
e. Is this apparent rotation clockwise or counterclockwise?
f. Explain using Fig. 3-4 how the marked blade's actual frequency, the frame rate, and the marked blade's observed frequency are related.
g. Repeat (a) to (f) when the propeller is turning at 57 rotations per second.