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Question

At a given instant, block A is moving upwards with velocity of 5 m/s. What will be velocity of block B at that instant ?
(Take upward velocity to be positive.)


A
15 m/s
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B
15 m/s
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C
5 m/s
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D
10 m/s
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Solution

The correct option is A 15 m/s

Since l1,l2,l3 & l4 are sections of the same string.
l1+l2+l3+l4=constant
dl1dt+dl2dt+dl3dt+dl4dt=0
VA+dl2dt+dl3dtVB=0 ....(1)
dl2dt is the velocity of pulley and it is attached to the same string with block A, velocity of pulley is same as velocity of block A.
i.e dl2dt=dl3dt=5 m/s
VA=5 m/s
Substituting in the constraint equation (1), we get
5+5+5VB=0
VB=15 m/s in downward direction
Since velocity in upward direction is positive VB=15 m/s

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