Question 1

A technician uses a stroboscope to “freeze” the rotation of a motorized wheel with a mark on it. With the mark appearing to stand still, the technician measures the strobe frequency at 10 Hz. Based on this measurement the technician determines the wheel’s speed to be 600 RPM:

A second technician comes along and disagrees with the first. According to the second, more testing is required before it can be sure that the wheel’s speed is actually 600 RPM. The first technician is perplexed: how can the speed be anything but 600 RPM, if 10 Hz is the strobe frequency where the mark is seen to “stand still?”


Question 2

Note that the LEDs shown in the schematic do not have voltage-dropping resistors connected in series. How is this possible, especially considering the author’s prototype circuit operated at 12 volts DC (far in excess of an LED’s continuous rating)?


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