Where can I get help with inclined plance friction problems online?

Question

The problem is:
Determine the maximum force P that can be applied to the a box weighing 400lb on an inclined plane. The incline plane is 20 degrees. The force P is applied pulling the box upward 30 degrees above the inclined plane. Static friction is 0.3.

Answer 1

The block slips when the parallel force Fp equals 0.3 * the normal force Fn.
Fn = g * m * COS(20) - P * SIN(30)
Fp = P * COS(30) - g * m * SIN(20)
Box slips when Fp = mu * Fn (mu = 0.3)
So the slip conditions are:
P * COS(30) - g * m * SIN(20) = mu * (g * m * COS(20) - P * SIN(30))
P * (COS(30) + mu * SIN(30)) = g * m * (SIN(20) + mu * COS(20))
P = g * m * (SIN(20) + mu * COS(20)) / (COS(30) + mu * SIN(30))
P = 2408.85 N.