Question

Two blocks with masses m and M are pushed along a horizontal frictionless furface by a horizontal applied force. The magnitude of the force either of these blocks has on the other is:

M being a bigger mass than m of course. I think the answer is:
mF/(M-m) or maybe MF/(M+m) but I'm not really 100% sure how to go about doing it. I know the forces are going to be equal though...

Answer 1

The force on the one closest to the force if F, now between them, the second block is experience a force =ma, and it is pushing back with that force. I don't know which one is in front.
what is a? F=(M+m)a or a= F/(m+M)
now as you indicate, put the mass of the one in front on top, and you have it.

Answer 2

Oh, the larger block (mass) is in front. The force is pushing on the large mass and the little mass is in front of the large mass.
Force -> Big Mass -> little mass

Acceleration isn't given, it's just asking the "magnitude of the force of either of these blocks on the other is".
The answers to the question don't include a variable for a; just M, m, and F. I read your explaination but am still a bit confused. I think the answer is MF/(M+m) though.