Finding the forces between two masses

[Dcarroll]

Homework Statement

Here is the problem with the picture

View attachment 3blocks.doc

Homework Equations

F=ma

The Attempt at a Solution

I drew the free body diagrams for each block. Basically I am solving for FM1 on FM3 and FM3 on FM1 (I know that they are equal). I figured that the Net Force is 3N so i can solve for the acceleration of the block, 3N=21kg x a...a=0.143m/s^2

Im kind of stuck now, i have the net force and the acceleration of the whole system but i cannot figure out the force that M3 acts on M1 and vice versa

 

[issacnewton]

Hi, you need to find out the internal forces, so equating net force on the system as a whole to the total mass x acceleration won't help. Set up the FBD for each block separately and then it will be easier to set up the equations of motion. for example, force on the block m₁ is F₁ to the right and F₃₁ (force exerted by the m₃ on m₁) to the left. and m₁g downwards and N₁ (normal reaction) upwards. there is no friction between the ground and the block m₁. set up similar FBD's for other two blocks