Momentum (Conservation Of) Question.?

1. Friction between cart wheels and the ground.

2. Material of the cart. They may not be perfect for collision. Some energy may be absorbed and heat can be produced.

3. The collision normally produces a sound that also takes away some energy.

4. If there are sparks, that may also take away energy.

These are some of the things that make the collisions not to be perfect elastic.

Collision itself. When it happens we can not observe moving objects as ideal points. They rather become rigid bodies.

During the collision, the biggest part of energy goes to the heat created in the moment of impact and bending of materials.

Friction of the wheels on the track

Air friction

In general: All forces applied to the carts that is not exerted by the carts. (So one cart exerting force on another is fine, while the air exerting a force on a cart is not).