A cubical block of mass M and edge a slides down a rougg inclined plane of inclination `theta` with a uniform velocity. The torque of the normal force on the block about its centre has magnitude.

Answer

A, B

Explanation

The force mg acting on the body has two components mg sin theta and mg costheta` and the body will exert a normal reaction R(say).
Since R and `mg costheta` pass through same line so there will be no torque due to R and `mg cos theta`. The only torque will be produced by `mgsintheta`.
`:. tau=fxr (r=a/2)`
(a=ages of the cube)
`:. tau=mgsintehtaxxa/2`
`=1/2mgasintheta`