Question

In order to measure the speed of a steam turbine which is higher than the flashing rate of stroboscope. The flashing rate was gradually reduced from the highest, noting all the speed at which single image is observed of a mark on the rotating object. These speeds are $2400,2000,1800$ and $1200$ rpm respectively. The actual speed of the steam turbine is ____ $rps$

• A. 120

Answer 1

The correct option is A 120
$\text{The actual speed is given by,}$

$n=\frac{f_m{f}_1(m-1)}{(f_m-f_1)}$

$\text{where}{f}_m=\text{highest flashing frequency}$

$f_1=\text{lowest flashing frequency}$

$m=\text{number of flashing frequecies}$

$n=\frac{[2400\times 1200](4-1)}{[2400-1200]}=7200rpm$

$n=\frac{7200}{60}=120rps$