3 Blocks being pushed together by a force

[JyN]

Homework Statement

Three blocks on a frictionless horizontal surface are in contact with each other. the 1st mass touches the 2nd, the 2nd touches the 3rd. A force F is applied to the 1st block. Draw a free body diagram for the system, and each block individually. Determine the net force and acceleration of the system. The net force on each block, and the force of contact on each block.

Homework Equations

Newtons 2nd law. F = ma

The Attempt at a Solution

I have a free body diagram for each block. for the first block, i have the force F acting, as well as the reaction from the 2nd block which is equal and opposite to F. On the 2nd block i have the force from the 1st block (which is = to F), and a negative reactive force from the 3rd block (-F). Finally, on the 3rd block there is a force from the 2nd one acting to push it forward.

The net forces on the 1st two blocks is 0, and the force on the 3rd block is F. Is this correct? from this, it seems that only the 3rd block moves, which is obviously not the case.

Is it right to think of it like this: The 3rd block moves forward an infintesimally small amount, and in the instant it comes out of contact with the 2nd block, there is no longer a reactive force from block 3 on block 2, so block 2 moves forward an infintesimally small amount (coming again in contact with block 3, but there is no force from block 2 applied to block 3 because there is no force from block 1 on block 2) , allowing block 1 to come back in contact with block 2, making the net forces on blocks 1 and 2 0, and causing block 3 to move forward another infintesimally small amount. This process continues on and the system appears to move forward in unison.

The final part of the question has me assign F = 96N, and each block weighs 12Kg. I would get an answer that has blocks 1 and 2 sitting still and block 3 accelerating forward. The question then asks if this makes sense intuitively, obviously it doesnt. But i can't use math to model what i described in the above paragraph. And i would simply take a shortcut and treat all three blocks as 1. Would that be correct?

tl;dr: Read the italicized paragraph

Thanks :)

 

[PhanthomJay]

Your error is that you are assuming that the forces on the first and 2nd blocks cancel, and thus the net force on these blocks is 0. This is wrong. The blocks are accelerating together, meaning each one is accelerating at the same rate as the system of blocks. So don't use Newton 1, use Newton 2...F_net =ma, for each block or set of blocks, to solve for the correct forces acting on them. And don't forget Newton 3..which is NOT the same as Newton1!