Physics
posted by Sbjjayo .
An object of mass 20kg slides down an inclined plane at an angle of 30 degree to the horizontal. The coefficient of static friction is?

If 30 degrees is the minimum angle that allows slipping to start, then the coefficient of static friction (mu_s) is given by
M*g*sin30 = M*g*mu_s*cos30
mu_s = tan30 = 0.5774
The mass M does not matter.
Angles greater than 30 degrees will also allow slipping. 
0.5774