IIT JEE , AIEEE Forum - Physics , Mechanics

kane

a body of mass 5kg strikes a body of mass 2.5kg initially at rest which after collision begins to move with K.E. of 5J.assuming the collision to be elastic,then K.E. of first body before impact is:

a 0.62J

b 5.62J

c 7.28J

d 4.20J

don't know the answer guys but this one is simple but i'm not getting any of these 4 answers so plz help me

rhd92781

ans shud be b yaar

rhd92781

let v n v1 be velocity of 5 kg mass b4 collision n after collision respectively.
v2 be velocity of 2.5 kg mass aftr collision.
Equate momenta.
5v=5v1+2.5v2
or 2v=2v1+v2.

1/2 * 2.5 * v2^2=5
or v2=2 m/s.
Now 2v = 2v1 + 2
or v=v1+1...............(i)

Equate KE.
1/2 * 5 *v^2 = 1/2 * 5 * v1^2 + 5
or v^2 = v1^2 + 2..............(ii)

Solve eqns (i) and (ii)...
v1^2 + 1 + 2v1 = v1^2 + 2
or v1=1/2
therefore, v=1/2 + 1 =3/2
so KE of 5 kg body b4 impact
= 1/2 * 5 * (3/2)^2
=5.625 J

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