Q.

Two blocks of masses 8 kg and 12 kg are connected at the two ends of a light inextensible string. The string passes over a frictionless pulley. The acceleration of the system is

1 g4  
2 g5  
3 g8  
4 g6  

Ans.

(2)

Let a be the common acceleration of the system and T be tension of the string.

The equations of motion of two blocks are

T-8g=8a                ⋯(i)

and  12g-T=12a        ⋯(ii)

Adding (i) and (ii), we get

4g=20a     or     a=g5