Question

A strobocope projects $6000$ flashes per minute on a disc with $10$ patterns mounted on the shaft of a machine. If these $10$ points appear to be revolving once in $12sec$, then the two possible shaft speeds in rpm are respectively.

• A. $595,695$
• B. $590,610$
• C. $600,600$
• D. $588,612$

Answer 1

The correct option is A $595,695$

Speed of shaft when pattern appears stationary

$n=\frac{f}{m}=\frac{6000}{10}=600rpm$

now, since $10$ points appears to be revolving once in $12sec$ hence the pattern move in forward or backward in the direciton of rotation at a speed of $5rpm.$

The speed of shaft when pattern move forward in the direction of rotation
$=600+5=605rpm$

Similarly, speed of shaft when pattern move backward in the direciton of rotation
$=600-5=595rpm$