physics
posted by Nelly .
Two planes of the same mass collide headon and becomes tangled so that they move on together. If the engines of both were stopped at the moment of impact and the speeds of the planes at impact were 120m/s and 200 m/s, find the joint velocity immediately after collision.

Apply the conservation of momentum law, using the appropriate sign for velocity. If the 120 m/s plane (#1) is going in the + direction, the 200 m/s plane (#2) is going in the  direction.
Each plane has mass M.
Momentum conservation requires:
MV1 MV2 = 2M*Vfinal
Vfinal = (1/2)(V1  V2) = 40 m/s 
thanx