The pipe of weight W is to be pulled up the inclined plane of slope α using a force P. If...

The pipe of weight W is to be pulled up the inclined plane of slope α using a force P. If P acts at an angle ϕ, show that for slipping P = Wsin(α + θ)/cos (ϕ − θ), where θ is the angle of static friction: θ = tan−1 µs.