The motion of a system is governed by the Momentum Principle. This principle describes how a system changes its motion when it experiences a net force. We observe that when objects move in a straight line at constant speed experience no net force. This observation is critical to our understanding of motion (This observation is often called "Newton's First Law of Motion").

In these notes, you will be introduced to the idea of a system, momentum, net force, and how a system's momentum and the net force it experiences are related. In another set of notes, you find a few useful formula for when the net force acting on a system is a constant vector (fixed magnitude and direction).

We can consider a single object or a collection of objects to be a “system.” Anything that we choose to not be in our system exists in the “surroundings.” In mechanics, we choose systems by considering what objects we want to predict or explain the motion of.

Through interactions with the surroundings, systems can change their momentum, energy, angular momentum, and entropy. For the time being, you will work with single-particle systems, and we will consider only how they change their momentum.