# A body of mass 3kg makes an elastic collision with another body at rest and continues to move in the original direction with a speed equal to one – third of its original speed. Fine the mass of the second body.CLASS - XI PHYSICS (Work, Energy and Power)

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by ashaRamroop

2015-09-25T19:16:50+05:30

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M1  = 3 kg      u1             v1   =  u1/3
m2  = ?          u2 = 0       v2   =  ?

Elastic Collision   =>    Conservation of Linear Momentum and Total Energy.

m1 u1 + m2 u2 = m1 v1 + m2 v2
3 u1 + 0 = 3 u1 /3 + m2 * v2
=> v2 = 2 u1/  m2

1/2 *3* u1² + 1/2 * m2 * 0 = 1/2 * 3* u1²/9 + 1/2 * m2 * v2²
u1² (8/3)  =  m2 * (2 u1 / m2)²
m2 = 4 * 3/8 = 1.5 kg

we can also find    v2  =  2 u1 /1.5 = 4 u1 /3

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