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in Laws of motion by (15.7k points)
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Comprehension Type 

Paragraph: An arrangement designed to measure the acceleration due to gravity at a place consist of two blocks A and B, of mass m and 2m respectively connected to each other by means of a light inextensible string passing over a light frictionless pulley as shown in the Figure. A light and very rough plank, rigidly held in position, supports the block A. The system, it is observed does not move at all. The portion of the string OA, is initially horizontal. Assume that the acceleration due to gravity, (g = 10m/s2)

(i) The net force due to plank, acting on the block A, has magnitude 

(A) 2mg 

(B) mg 

(C) 3mg 

(D) 5mg 

(ii) The magnitude of the force exerted on the pulley by the clamp is, when the system is in equilibrium.

(A) 4mg 

(B) 4mg / 3 

(C) \(\frac{2 \sqrt 2 mg}{3}\)

(D) 2 \(\sqrt{2}\)mg

(iii) The plank is suddenly broken by an impulsive force, acting downwards. The instantaneous accelerations of A and B, just after the plank is removed, are respectively, 

(A) 10 m/s2 and 10 m/s2

(B) 20 m/s2 and 3.33 m/s2

(C) 12 m/s2 and 6.66 m/s2 

(D) None of the above

2 Answers

+1 vote
by (39.0k points)
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Best answer

(i) (D) 5mg

(ii) (D) 2√2 mg

(iii) (C) 12 m/s2 and 6.66 m/s2

Given that the plank has very rough surface.

Net Contact force acting between block A and plank

Now just after this instant;

Normal reaction becomes zero.

On body A;

+1 vote
by (15.2k points)
edited by

(i) (D) 5mg 

(ii) (D) 2 \(\sqrt{2}\)mg

(iii) (C) 12 m/s2 and 6.66 m/s2 

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