The two blocks shown in the diagram are at rest on a horizontal surface when a force P is applied to block B. Blaocks A and B have masses 20kg and 35kg respectively. The coefficient of friction between the two blocks is 0.35 and between the horizontal surface and Block B is 0.3.
Determine the maximum force, P, before A slips on B.
F = ma , Friction max = uR
The Attempt at a Solution
Okay this thing has been killing me for months.
Friction between B and the ground is (20 + 35)g(0.3) = 16.5g and between B and A is
(0.3)(35)g = 10.5g. This produces anti-friction which propells A forward. Friction between A and B is (20)(0.3)g = 6g.
So, P should equal 16.5g +10.5g + (10.5g - 6g) = 31.5g N.
I'm supposed to be getting 35.75g N.
Where am I going wrong?