# Dynamics Question Involving two blocks on a plane

## Homework Statement

http://i.imgur.com/K2hYHgq.png
Picture here with question.

F=ma

## The Attempt at a Solution

For the system to begin moving, a>0 therefore

Fnet>0

I drew some FBDs and came up with four relevant forces total; the force of gravity along the plane for each of the two blocks, and the force of friction on the plane by m1 and m2, and the force of friction on m1 by m2. I ended up with something like this (g is cancelled):

m2sinx - m1 sinx - um1cosx - u(m1+m2)cosx > 0

tan x > (2um1+um2)/(m2-m1)

And solved for x from there.

However, my answer is wrong. I think I'm getting stuck with the forces of friction, the denominator in my expression (m2-m1) seems very reasonable considering the system should act similarly to a block of mass m2-m1 with the extra force of friction.

## Answers and Replies

Filip Larsen
Gold Member
Welcome to PF!

If your equation is from a FBD of m1 you may want to reconsider which mass is involved in the expression for the frictional force. (hint: if the system is not moving, can the force of friction then depend on m2?)

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