a 5-kg crate is resting on a horizontal plank. the coefficient of static friction is .5 and the coefficient of kinetic friction is .4. after one end of the plank is raised so the plank makes an angle of 25 degrees with the horizontal, the force of friction is?

Answer 1

Net force down plank if it is stationary:

mgSin25-.5cos25 if this is positive, then the block is moving, and the coeff friction is .4.

However, it is negative, so it is still standing still.