Systems
Conservation of Momentum
In this lesson and the next set of lessons, we are going to look at momentum when two objects interact with each other and, specifically, treat both objects as one system. To do this properly we first need to define what we mean we talk about a system, then we need to look at what happens to momentum overall and we will explore the applications of momentum in these interactions.
Systems
Definition: System
A system is a physical configuration of particles and or objects that we study.
For example, earlier we looked at what happens when a ball bounces off a wall. The system that we were studying was just the wall and the ball. The wall must be connected to the Earth and something must have thrown or hit the ball but we ignore those. A system is a subset of the physical world that we are studying. The system exists in some larger environment.
In the problems that we are solving we actually treat our system as being isolated from the environment. That means that we can completely ignore the environment. In reality, the environment can affect the system but we ignore that for isolated systems. We try to choose isolated systems when it makes sense to ignore the surrounding environment.
Definition: Isolated system
An external force is a force acting on the pieces of the system that we are studying that is not caused by a component of the system.
It is a choice we make to treat objects as an isolated system but we can only do this if we think it really make sense, if the results we are going to get will still be reasonable. In reality, no system is competely isolated except for the whole universe (we think). When we look at a ball hitting a wall it makes sense to ignore the force of gravity. The effect isn't exactly zero but it will be so small that it will not make any real difference to our results.
This lesson is part of:
Momentum and Impulse