At a given instant, block A is moving upwards with velocity of 5m/s. What will be velocity of block B at that instant ?
(Take upward velocity to be positive.)
A
−15m/s
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B
15m/s
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C
−5m/s
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D
−10m/s
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Solution
The correct option is A−15m/s
Since l1,l2,l3&l4 are sections of the same string. l1+l2+l3+l4=constant ⇒dl1dt+dl2dt+dl3dt+dl4dt=0 ⇒VA+dl2dt+dl3dt−VB=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=5m/s VA=5m/s
Substituting in the constraint equation (1), we get 5+5+5−VB=0 VB=15m/s in downward direction
Since velocity in upward direction is positive ∴VB=−15m/s