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}}.