Elastic: Kinetic energy is conserved, objects bounce off each other. Inelastic: Kinetic energy is not conserved, energy is lost as heat/sound/deformation, objects may stick together.

Momentum ($\vec{p}$) is a measure of how much "oomph" an object has in motion. It's the product of an object's mass ($m$) and its velocity ($\vec{v}$).

Change in momentum ($\Delta \vec{p}$), also known as impulse ($\vec{J}$), is the result of a force acting over a period of time. It's the difference between the final and initial momentum.

Impulse is equal to the change in momentum. It's also equal to the force ($\vec{F}$) multiplied by the time interval ($\Delta t$) over which the force acts: $\vec{J} = \Delta \vec{p} = \vec{F}\Delta t$

In a closed system (no external forces), the total momentum remains constant. This means the total momentum before a collision equals the total momentum after the collision: $\vec{p}_{initial} = \vec{p}_{final}$

A collision in which kinetic energy is conserved. Objects bounce off each other without losing energy (idealized).

A collision in which kinetic energy is not conserved. Some energy is lost as heat, sound, or deformation. Objects may stick together (perfectly inelastic).

1. Calculate initial momentum of each object. 2. Calculate total initial momentum. 3. Apply conservation of momentum. 4. Solve for the unknown.