Question 249: Centre of Mass and Rotational Motion

Question

A metal coin of mass 5 g and radius 1 cm is fixed to a thin stick AB of negligible mass as shown in the
figure. The system is initially at rest. The constant torque, that will make the system rotate about AB at 25
rotations per second in 5s, is close to :
![](https://imagex.cdn.examgoal.net/7kejek19p8mp8/f2e092be-211c-448c-b48d-fbe513359c21/cdc752c0-e54f-11e9-a7bc-b7b08ce8d541/file-7kejek19p8mp9-322w.jpg)

Accepted Answer

m = 5 ÃÂ 10
Ã¢ÂÂ3
kg, r = 10
Ã¢ÂÂ2
m
$\omega $ = 25 ÃÂ 2$\pi $ rad/5
= 50 $\omega $ rad/sec
$\omega  = {	au  \over I}t$
$	au  = {{I\omega } \over t} = {{5m{r^2}} \over 4} 	imes {\omega  \over t}$
$ = {{5 	imes 5 	imes {{10}^{ - 3}} 	imes {{10}^{ - 4}} 	imes 50\pi } \over {4 	imes 5}}$
$ = {{25\pi } \over 4} 	imes {10^{ - 6}} = 2 	imes {10^{ - 5}}$ Nm

Answer

7.9 × 10
–6
Nm

Answer

4.0 × 10
–6
Nm

Answer

2.0 × 10
–5
Nm

Answer

1.6 × 10
–5
Nm

Centre of Mass and Rotational Motion

A metal coin of mass 5 g and radius 1 cm is fixed to a thin stick AB of negligible mass as shown in the figure. The system is initially at rest. The constant torque, that will make the system rotate about AB at 25 rotations per second in 5s, is close to :

Choose the correct answer: