### Two Blocks with a Spring

Two Blocks with a Spring
(a) The total force on the second block:
$\sum F_{x}=m_{2}a_{2x}=(3.8kg)(2.6\frac{m}{s^{2}})=9.9N.$
There is only ONE important force on the block: the force of block
1 on block 2.

(b) There is only one important force on block 1. This is the force
of block 2 on block 1. From Newton's third law, the magnitude (size)
of this force is 9.9N. Now use Newton's second law:
$\sum F_{x}=-9.9N=m_{1}a_{1x}=(4.6kg)a_{1x}.$
So $a_{1x}=-2.2\frac{m}{s^{2}}$ at the same moment that $a_{2x}=2.6\frac{m}{s^{2}}$.