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Question

As shown in the figure, the 2 kg block is subjected to an external force of 42 N. What is the value of normal reaction (N) acting on the 3 kg block (exerted on it by the surface), when spring reaches maximum extension? (Take g=10 m/s2)


A
20 N
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B
30 N
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C
8 N
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D
5 N
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Solution

The correct option is C 8 N
Draw the FBD of both blocks:


When spring reaches maximum extension, acceleration of block of 2 kg=0
Simillarly for block of 3 kg, since it has not broken contact with ground, it's acceleration=0

For 2 kg block: 4220kx=0......(1)
or kx=22

Similarly for 3 kg block:
N+kx30=0......(2)

Putting the value of kx from equation (1) in equation (2), we get
N=3022=8 N

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