Question

Two blocks of mass 2kg are connected by
massless ideal spring constant k=10N/m. The
upper block is suspended from roof by a light
inextensible strings A. The spring is initially un-
stretched. The lower block given an initial velocity
2m/s downwards. The minimum tension
developed in the string connected to the ceiling is
nearest to:
(1) 10 N
(3) 18.1 N
(2) 20 N
(4) 21 N​

Answer 1

(2) 20 N

Explanation:

Given: Two blocks of mass 2kg are connected by massless ideal spring constant k = 10N/m. The upper block is suspended from roof by a light inextensible strings A. The system shown in equilibrium. The string A is now cut, the acceleration of upper block just after the string A is cut will be g = 10 m/s2.

Find: The minimum tension developed in the string connected to the ceiling.

Solution:

Please refer to the attached picture for the diagram.

Now, just after the spring A is cut, free body diagram of upper block.

So 4g = 2a

a = 4/2g = 2g = 2 * 10 = 20 m/s2

Option 2 is correct.