# Torque problem

1. Jan 15, 2010

### sachin123

1. The problem statement, all variables and given/known data

cubical block of mass m ,edge a,slides down the inclined plane of inclination $$\varphi$$ with uniform velocity.
torque of normal reaction on the block about its centre is?

3. The attempt at a solution
Torque=mgcos$$\varphi$$ X a/2
mgcos$$\varphi$$sin$$\varphi$$a/2
Book says:
mgsin$$\varphi$$a/2

2. Jan 15, 2010

### ideasrule

Think about it this way: gravity acts on the center of mass, so gravity contributes no torque. Friction does, and the normal force has to balance the torque that friction creates.

3. Jan 15, 2010

### sachin123

So that should make the answer mgcos$$\varphi$$sin$$\varphi$$a/2
and not
mgsin$$\varphi$$a/2
Right?