Inelastic collision causes loss of momentum. What are two physical reasons for the loss?

In physics lab we collided two objects that then both started moving in the same direction. We measured the velocities through two photo gates. We know momentum was lost because the velocities decreased after the collision. What are two physical reasons for the loss of momentum?

The Conservation of Energy

A skier coasts down a very smooth, 10-m-high slope (v =20 m/s), what is his speed at the bottom of the slope. If the skier has a mass of 60kg and the force of friction retards his motion by doing 2500 j of work, what is his speed at the bottom of the slope?

Work Done by Constant Force

A 500-kg helicopter ascends from the ground with an acceleration of 2.00m/s2. Over a 5.00-s interval, what is (a) the work done by lifting force, (b) the work done by the gravitaional force, and (c) the new work done on the helicopter?

Power

Water is lifted out of a well 30.0m deep by a motor rated at 1.00 hp. Assuming 90% efficiency, how many kilograms of water can be lifted in 1 minute?

Angular Measure

Assume that the Earth's orbit around the sun is circular. In kilometers, what is the approximate orbital distance the Earth travels in three months?