A small box is held in place against a rough vertical wall by someone pushing on it with a force directed upward at 28degrees above the horizontal. The coefficients of static and kinetic friction between the box and wall are 0.40 and 0.30, respectively. The box slides down unless the applied force has magnitude 18 N.

Question

What is the mass of the box in kilograms?

Answer 1

Here's a free-body diagram indicating all forces acting on the box.
http://img142.imageshack.us/img142/7156/leahi.jpg
Summing forces in the x-direction will let you solve for the value of the normal reaction, N.
Using N and the static friction coefficient (since the box has not moved yet), you will be able to find the mass of the box. If I am not mistaken, the mass is around 1.51 kg.
Post if you have questions.