Here's why:

* Force: A push or pull that can cause a change in an object's motion.

* Weight: The force of gravity acting on an object's mass.

* Air friction (resistance): A force that opposes the motion of an object through the air.

* Momentum: A measure of an object's mass and velocity. It's not a force, but rather a property of an object in motion.

Momentum describes how much "motion" an object has, but it doesn't directly cause changes in motion like forces do.