Force between two contacting masses on a frictionless surface pushed from left or right The figure shows two boxes, with $m_1 > m_x$ that are on a level frictionless surface. We can apply a horizontal force $F$ either toward right on $m_1$ or toward left on $m_2$. The magnitude of the forces that the boxes exert on each other is
The answer is  "larger if $F$ is applied toward the left (which is on side of $m_1$ box) 
Why is this so? Shouldn't it be the same?
 A: 
We'll call the acceleration of the two boxes $a$. They have the same acceleration $a$ because they're moving together. We know from Newton's second law that for any mass $m$ the force is related to the acceleration by $F = ma$.
The force required to push the two boxes (labelled $F$ in the diagram) is given by:
$$ F = (m_1 + m_2)a $$
and that force is the same whether you're pushing from the left or the right because in both cases you're pushing both boxes so the total mass you're pushing is the same.
When you push from the right (top diagram) you push the small box with force $F$ and the small box pushes the big box with force $f$. When you push from the left (lower diagram) you push the big box with force $F$ and the big box pushes the small box with force $f$. The force $f$ is different in the two cases because the force to push just the big box is different from the force required to push just the little box. You can use Newton's second law again to calculate the value of $f$ in the two cases.
