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# 4: Linear Momentum

• 4.1: Repackaging Newton's Second Law
Our only experience with Newton's second law is as a description of the reaction of an object's velocity to a force. Here we take a look at the second law in a new light that we will soon find useful.
• 4.2: Center of Mass
We discussed of center of mass in the context of Newton's second law without going into detail about what it is. It's now time to remedy that.
• 4.3: Momenta of Systems
The power of our repackaging Newton's second law in terms of momentum starts to become clear when we apply it systems of objects.
• 4.4: Momentum and Energy
We now have two physical quantities that exhibit conservation laws: energy and momentum. Here we look at their interplay, and get new insight into the work-energy theorem.
• 4.5: Collisions
Momentum conservation is especially well-suited to dealing with collisions, because the forces between the colliding objects (which are typically quite complicated) can be ignored when it comes to momentum conservation of the system.
• 4.6: Problem Solving
Let's see how we can apply momentum conservation to some classic problems.

Thumbnail: A pool break-off shot. Image used with permission (CC-SA-BY; No-w-ay).