A fan has four identical, symmetrically placed blades. The blades are rotating clockwise at twenty revolutions per second. A) What is the smallest time interval between stroboscope flashes that will make the fan blades appear motionless?

B) What is the highest frequency (in flashes per second) at which a stroboscope will make the
fan blades appear to stand still? Show your calculation.

C) The same questions as (a) and (b), but someone has put a yellow dot on one blade, and now you want the yellow dot to appear to be standing still. Explain, and show your calculation.

D) Now the stroboscope is set for nineteen flashes per second, and the yellow dot appears to be slowly rotating. Which direction does it appear to rotate, clockwise or counterclockwise? Explain, and show your calculation.

E) The same as (d), but the stroboscope is set for twenty-one flashes per second. Explain, and show your calculation.