# Physics

A girl standing on a stationary uniform plank 10m long. The plank can move with negligible friction along the surface of the water.

A) the girl walks along the plank with a velocity of 2ms-1 relative to the plank until she reaches the end of the plank. The centre of mass of the system is now 2m from the end of the plank. Calculate the mass of the girl is the mass of the plank is 12.6kg.

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1. The center of mass of the system continues at rest because there are no external forces on the system.

cg at 0 forever
m girl * distance from 0 = m plank * distance from 0
center of plank moves 3 to right if girl now 2 meters from CG of system to left
3*12.6 = 2 m
m = 3 * 6.3 = 18.9 kg

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posted by Damon

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