Determine the relationship that governs the velocities of four cylinders if vA, vB, vC and vD represents velocities of block A, B, C and D. Consider downward velocity as positive and strings inextensible.
A
4vA+8vB+4vC+vD=0
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B
−4vA+8vB−4vC+vD=0
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C
4vA−8vB+4vC−vD=0
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D
4vA+8vB−4vC+vD=0
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Solution
The correct option is A4vA+8vB+4vC+vD=0 Let the velocities of pulleys P1, P2, P3, and P4 be vP1, vP2, vP3 and vP4 respectively. vp2=vC and vp4=vB. All velocities are assumed to be in downward direction. (a) l1+l2+l3=constant Differentiating w.r.t time, we get ⇒vD+vp1+vp1=0 ⇒2vp1=−vD ... (i) (b) l4+l5+l6+l7=constant Differentiating w.r.t time, we get −vp1+vC+vC+vp3+vp3=0 ⇒2vp3+2vC=vp1 ... (ii) (c) l8+l9+l10=constant Differentiating w.r.t time, we get −vp3+vB+vB+vA=0 ⇒2vB+vA=vp3 ... (iii) From (i), (ii) and (iii) 4vB+2vA+2vC=−vD2 ⇒4vA+8vB+4vC+vD=0