## College Physics (7th Edition)

$3.0\,m/s^{2}$
According to Newton's second law, $F=ma$ $\implies a=\frac{F}{m}=\frac{F}{m_{1}+m_{2}}=\frac{F}{\frac{F}{a_{1}}+\frac{F}{a_{2}}}$ $=\frac{1}{\frac{1}{a_{1}}+\frac{1}{a_{2}}}=\frac{a_{1}\times a_{2}}{a_{1}+a_{2}}=\frac{4.0\,m/s^{2}\times12\,m/s^{2}}{4.0\,m/s^{2}+12\,m/s^{2}}=3.0\,m/s^{2}$