I had this problem on a test I took last week, which our professor recently returned to us. Everyone has been getting the answer 3.33, but I'm not sure if it's correct. I got it wrong on the test. Here is the problem: Given: M1: 5kg Theta: 60* Coefficient of static friction: 0.4 Excuse the crude diagram, but it was either that or my sloppy handwriting from my test. Anyway, without further ado, the objective of the problem is: Find the minimum mass M2 such that the blocks do not move. Here's my force analysis. M2 vertical forces: T - M2g = 0 M1 horizontal forces: M1g[sin(theta)] - T - fs = 0 M2 vertical forces: N - M1g[cos(theta)] = 0 fs = mu static * N fs = (.4)(24.5) = 9.8 Then, setting the two tensions equal to each other, I get: M2g = M1g[sin(theta)] - fs M2 = (M1g[sin(theta)] - fs) / g After subbing all the numbers in, I get M2 = 3.33; according to one kid, he put 3.33 on his test but got it marked wrong. I really don't know any other way of doing this problem. Any help would be great.