QUESTION 3 The same 24 inch fan is now pulsed at 755 flashes

QUESTION 3

The same 24 inch fan is now pulsed at 755 flashes/minute. When you observe the fan under the stroboscope you see:

The fan is stationary.

The fan rotating clockwise

The fan rotating counter-clockwise

Multiple blades

QUESTION 4

A tachometer is an instrument that can detect the speed of a rotating object. According to the tachometer on a machinist’s lathe, it is spinning clockwise at 1255 RPM. You attempt to confirm the RPM using a stroboscope and find that when pulsed at 1255 flashes/minute the lathe rotates slowly in the clockwise direction. What is the speed of the lathe?

1255 RPM

Faster than 1255 RPM

Slower than 1255 RPM

Not enough information

Solution

Question 2) D

The stroboscope speed is 4 times the fan speed. Hence, for every rotation of the fan blade the strobe flashes 4 times. Thus we will see 4 blades.

Question 3) C

The strobe flash rate is slightly higher than the fan rotating speed. Thus the fan would not be able to complete one complete rotation. At the time of the second flash, the fan blade would just about reach its starting position. Thus it would appear to be moving counter-clockwise.

Question 4) B

The strobe flash rate is set at 1255 RPM clockwise.But the fan rotates more than 360^o before the second flash. Thus the rotation speed of fan is slightly more than strobe flash rate.