Question

A table fan rotating at a speed of 2400 rpm is switched off and the resulting variation of revolution/minute with time is shown in figure. The total number revolutions of the fan before it, comes to rest is

• A. 160
• B. 280
• C. 380
• D. 420

Answer 1

Answer: (B)

280

Given that,

Speed of rotation of table fan$=2400rpm$

So after switched off the fan,the total number of revolution of the fan before it,comes to rest is,

Total number of revolutions = area under n-t graph
$=\frac{1}{2}\times 8\times \frac{1800}{60}+8\times \frac{600}{60}+\frac{1}{2}\times 16\times \frac{600}{60}$
$=120+80+80=280$

so after 280 revolution fan will come in rest.