physics help needed...

Question

A horizontal force of 10 N is just sufficient to hold the box  stationary against  the wall, coefficient of friction between the block and the wall is 0.2. weight of the block is ?   

1. 20N
2. 50N
3. 100N
4. 2N

Answer 1

The answer is 20N, #1

 

The force against the box is 10N. This force is the normal force between the box and the wall. The force of friction is the coefficient of friction times the normal force.

 

If there is no movement in the y direction (F= ma), then acceleration is zero and all the forces must add to zero. Thus:

 

Friction - weight = 0

10*.2 -weight = 0

weight = 20N