Two objects of mass m and 2m are travelling in opposite directions with the same speed v. The objects collide and stick together. What is the kinetic energy lost in the collision?

Answer 1

conserving momentum,
mv - 2mv = 3mv'
v' = -v/3
KE lost is thus
(1/2 mv^2 + 1/2 * 2m (-v)^2) - 1/2 (3m)v'^2
...