1.

A moving particle of mass m makes a head on elastic collision with a particle of mass 2 m which is initially at rest. What fraction of energy of the colliding particle is lost after collision ?

Answer»

`8//9`
`7//8`
`4//5`
`2//3`

Solution :Here by conservation of MOMENTUM
`mu+0=mv_1+2 mv_2` or `u-v_1=2 v_2`
ALSO by conservation ENERGY
`1/2"mu"^2=1/2mv_1^2+1/2(2m)v_2^2`
or `u^2=v_1^2=2v_2^2` or `(u-v_1)(u+v_1)=2 v_2^2`
From these eqns.`u+v_1=v_2`.Now eliminating `V-2`, we GET `v_1=1//3u`
Fraction LOST
=`(1/2"mu"^2-1/2mv_1^2)/(1/2mu^2)`
=`(u^2-(-1//3u)^2)/(u^2)=(1-1/9)=8//9`


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