When a cart with a certain mass (m) travels with a certain velocity (V) along a track, it is said to have a certain momentum (p) equal to its mass multiplied by its velocity. When it encounters another cart, it is observed that the sum of the momentums of these two carts does not change. If the carts "stick" somewhat then the collision is called inelastic and the velocity of the two cart system will be given by the sum of the momentums of the carts divided by the total mass of the system.
Win air*dw
dAl= -------------------------
Win air-Win water