The masses of `10kig` and `20kg` respectively are connected by a massless spring as shown in figure. A force of `200N` acts on the `20kg` mass. At the instant shown, the `10kg` mass has acceleration `12m//sec^(2)` . What is the acceleration of `20kg` mass?

Explanation

Equation of motion for `m_(1)` is `F=m_(1)a_(1)=10xx12=120N`. Force on 10kg-mass is 120N to the right. As action and reaction are equal and opposite, the reaction force F on 20kg mass F=120N to the left.
`therefore` Equation of motion of `m_(2)` is `200-F=20a_(2)`
`rArr200-120=20a_(2)rArr20 a_(2)=80rArra_(2)=(80)/(20)=4ms^(-2)`