Answer to Question #115459 in Classical Mechanics for Lizwi

Question

A brick is resting on a rough wooden plank that is inclined at an angle theta relative to the horizontal. The friction force acting on the brick, along the incline, is
1 : zero.
2 : equal to the weight of the brick.
3 : greater than the weight of the brick.
4 : less than the weight of the brick.

Answer 1

It's less then the weight of the brick, because of fricsion force is equal to projection of gravity forve on inclined plane and as each projection is less than force itself, variant 4 is correct