## College Physics (4th Edition)

The block on the left exerts a force of $\frac{F}{3}$ on the block to its right and this force is directed to the right.
We can find the acceleration of the system of both blocks: $F = (m+2m)~a$ $a = \frac{F}{3m}$ We can find the force $F_p$ with which the block on the right pushes the block on the left: $F_p = ma = m~(\frac{F}{3m}) = \frac{F}{3}$ By Newton's third law, the block on the left exerts a force of the same magnitude on the block to its right. The block on the left exerts a force of $\frac{F}{3}$ on the block to its right and this force is directed to the right.