1. Momentum is always conserved when collisions occur. Momentum is defined as being a quantity of motion, and it is a product of mass and velocity. A small object travelling with a high velocity has great momentum ( Ex. A bullet ), and a massive object travelling with a low velocity has great momentum ( Ex. A dump truck ).
2. Energy is always conserved. Kinetic energy is conserved in elastic collisions, but it is not conserved during inelastic collisions ( Ex. A collision that involves deformation of an object loses kinetic energy to heat, sound, light, etc. ).
3. Momentum in two dimensions can be linear, angular or some combination of the two.
4. If a direct collision occurs between masses that can be considered point masses, linear momentum is transferred to a system and conserved ( Ex. Billiard balls ).
5. If a projectile collides with a mass at some distance from a fixed end, angular momentum will be transferred to ( and conserved by ) the system ( Ex. A pendulum that is struck by a projectile ).
6. When a projectile collides with an object away from its center of mass, and the object’s motion isn’t restricted in any manner, both linear and angular momentum will be transferred to ( and conserved by ) the system ( Ex. A kicked football both spins and moves forward linearly ).
7. An object that rotates freely will do so about its center of mass ( Ex. A boomerang or even a tennis racket that is thrown ).
RENAISSANCE PHYSICS 