Question

A ball of mass m moving at speed v0 collides
head on with a stationary ball of mass 3m.
If the collision is completely inelastic, what
is the ratio
Kf
Ki
of the kinetic energies of the
balls before and after the collision?
1.
1/2
2.
1/9
3.
1/3
4. 1, since Ki = Kf for any collision in which
no external forces act.
5.
4/16
6.
1/4
7. Zero, since a completely inelastic collision
does not conserve energy.
8.
9/16
9.
1/16
10.
2/9

Answer 1

@dan815 I don't know why, my answer is not one of them.
before collision, K_i = 1/2 mv^2
after collision, K_f = 1/2 (m +3m) v^2 = 2mv^2
so, the ratio K_f/K-i =4 which is not one of the choices. why? what's wrong with my logic?

Answer 2

not sure... but the answer is 1/4 :)