Consider the system shown in fig. the system is released from rest, find the tension in the cord connected between 1kg and 2 kg blocks.

Options

A. If `M gt m`
B. If `M gt 2m`
C. If `M gt m//2`
D. For any value of M

Answer

D

Explanation

Let spring does not get elongated, then net pulling force on the system is `Mg+mg-mg` or simply Mg. Total mass being pulled is `M+2m`. Hence, acceleration of the system is
`a=(Mg)/(M+2m)`
Now since `a lt g` there should be an upward force on M so that its acceleration becomes less than g. It means there is some tension developed in the string. Hence, for any value of M spring will be elongated.