# Homework Help: Friction ( Inclined plane)

1. Aug 6, 2011

### NumberTheory

1. The problem statement, all variables and given/known data
A body of weight $$W$$ rests on a rough inclined plane and a force $$P$$ acting at angle $\alpha$ with the inclined plane just prevents the body from sliding down . If the inclined plane makes an angle $\phi$ with the horizontal , prove that

$$P = W\frac{sin(\phi - \lambda)}{cos(\alpha + \lambda)}$$

where $\lambda$ is the angle of friction.

2. Relevant equations

$$F = \mu N$$

3. The attempt at a solution

I just don't know where to start . Any hints would be appreciated.

2. Aug 6, 2011

### Staff: Mentor

Start with determining how $\lambda$ is related to $\mu$. Then use a Free Body Diagram to identify all the forces involved, and how they must combine to achieve a static condition (no motion for the block). Solve for P.

There will be some simple trig identities involved in simplifying the expression for P.

3. Aug 7, 2011

### NumberTheory

Thanks gneill , I finally proved it !