JEE Main · 2015 · 04 Apr 2015 · Offline · Laws of Motion

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Given in the figure are two blocks A and B of weight 20 N and 100 N respectively. These are being pressed against a wall by a force F as shown. If the coefficient of friction between the blocks is 0.1 and between block B and the wall is 0.15, the frictional force applied by the wall on block B is

(A)80 N
(B)120 N✓ Correct
(C)150 N
(D)100 N

Answer: (B)

Solution

Block

A

is held up only by friction from

B

, so that friction must carry

A

's whole weight,

f_{A} = 20

N upward. By Newton's third law

A

drags down on

B

with

20

N. Now balance

B

vertically: the wall's friction has to support

B

's own

100

N plus the

20

N that

A

hangs on it, so

f_{wall} = 100 + 20 = 120

JEE Main · 2015 · 04 Apr 2015 · Offline · Single correct option

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