First slide

Mechanical energy conservation

Question

System shown in figure is released from rest. Pulley and spring are mass less and the friction is absent everywhere. The speed of 5 kg block when 2 kg block leaves the contact with ground is (take force constant of the spring K = 40 N/m and $g = 10 m / s^{2}$ )

Difficult

A

$\sqrt{2} m / s$
B

$2 \sqrt{2} m / s$
C

$2 m / s$
D

$4 m / s$

Solution

$w h e n 2 k g m a s s l e a v e s t h e g r o u n d t e n s i o n i n t h e s t r i n g 2 g = k x = 40 x x = 0.5 f r o m \cos e r v a t i o n o f e n e r g y \frac{1}{2} k x^{2} + \frac{1}{2} M v^{2} = M g x w h e r e M = 5 k g$

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