Question

1. A piece of rectangular block weighing 5 kg is pressed on a vertical wall by a horizontal force applied to the block. If the coefficient of friction between the block and the wall is 2.0, what force is necessary to keep the block in place?

2. If the force in Prob. 1 is applied to the block such that it makes 60° with wall, what force is needed to keep the block in place?

Answer 1

down force = m g
up force = 2 F
2 F = 5 (9.81)
F = 5(9.81)/2

Part 2, 60 up or 60 down?

if 60 up

down force = m g

up force = F cos 60 + 2 F sin 60
= .5 F + 1.73 F = 2.23 F
so

2.23 F = 5 * 9.81

F = 5 (9.81) / 2.23