physics
posted by Nelly .
A railway track travelling along a level at 9 m/s collides with, and becoms coupled to a stationary truck. Find the velocity of the coupled trucks immediately after the collision if the stationary truck has a mass which is :
A ) equal to the mass of the moving truck
B) twice the mass of the moving truck

I assume that your 9 m/s speed refers to a "truck" (railcar) on the track. You wrote that the track is moving.
In both cases, apply the law of conservation of momentum. Let M be the mass of the initially moving truck
In the first case,
M*V1 = (2M)*Vfinal
Vfinal = (V1)/2 = 4.5 m/s
In the second case,
M*V1 = (M + 2M)*Vfinal = 3M*Vfinal
Vfinal = ___
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