JEE Main & Advanced
Sample Paper
JEE Main Sample Paper-35
question_answer
In the arrangement shown in the figure ends P and Q of an unstretchable string move downwards with uniform speed U. Pulleys A and B are fixed. Mass M moves upwards with a speed.
A) \[2U\cos \theta \]
B) \[U/\cos \theta \]
C) \[2U/\cos \theta \]
D) \[U\cos \theta \]
Correct Answer:
B
Solution :
\[{{l}^{2}}={{a}^{2}}+{{y}^{2}}\] \[2l.\frac{dl}{dt}\,=0+2y\,\frac{dy}{dt}\] (a is constant) \[-\frac{dy}{dx}=\frac{l}{y}\,\left( -\frac{dl}{dt} \right)\] \[or\,\,+{{v}_{M}}\,=\frac{1}{\cos \theta }\,(U)\,=\frac{U}{\cos \theta }\]