A point mass of 1kg collides elastically with a stationary point mass of 5 kg. After their collision, the 1 kg mass reverses its direction and moves with a speed of 2 ms –1 . Which of the following statement(s) is/are correct for the system of these two masses ?
Text Solution
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(a, c)
Since collision is elastic, so e = 1
Velocity of approach = velocity of separation
So, u = v + 2 .............(i)
By momentum conservation :
1 × u = 5v – 1 × 2
u = 5v – 2
v + 2 = 5v – 2
So, v = 1 m/s
And u = 3 m/s
Momentum of system = 1 × 3 = 3 kgm/s
Momentum of 5kg after collision = 5 × 1 = 5 kgm/s
So, kinetic energy of centre of mass = 
(m 1 + m 2 ) 
= 
(1 + 5) 
= 0.75 J
Total kinetic energy = 
× 1 × 3 2 = 4.5 J.
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