First slide

In the frame of centre of mass

Question

Two blocks of masses 5 kg and 2 kg are placed on a frictionless surface and connected by a spring. An external kick gives a velocity of 14 m/s to the heavier block in the direction of lighter one. The magnitudes of velocities of two blocks in the centre of mass frame after the kick are respectively:

Moderate

A

4 m/s, 4 m/s
B

10 m/s, 4 m/s
C

4 m/s, 10 m/s
D

10 m/s, 10 m/s

Solution

$v_{cm} = \frac{m_{1} v_{1} + m_{2} v_{2}}{m_{1} + m_{2}} = \frac{2 \times 0 + 5 \times 14}{2 + 5} = 10 m / s$
$v_{1} - V_{cm} = 14 - 10 = 4 m / s$
$v_{2} - V_{cm} = 0 - 10 = - 10 m / s$

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