Read Collisions in One Dimension
On pages 468 to 470 of your physics textbook, you will find an introduction to collisions of systems of objects.



A system is a collection of two or more objects. For example, a collection of curling stones on a sheet of ice would be considered a system.

If this system was an isolated system this would mean that no net external force acts on the system and no mass or energy enters or leaves the system. In our curling system this would be mean that the number of stones does not change and not outside forces, such as the players pushing the stones would occur. Likewise, energy would not enter or leave such as heat energy loss due to friction of stones moving across the ice.

The opposite of an isolated system is a non-isolated system of which there consists of two types, open and closed. A closed system is one where no mass enters or leaves the system but objects within the system can be influenced by outside forces and energy can enter or leave the system. Our curling system would be considered a closed system if no stones were added or removed, but the stones could be influenced by outside forces, and energy could enter or escape.


By contrast, an open system is one where mass and energy may enter or leave the system, and external forces may influence the system.  This would mean for our curling rink example, stones may be added or removed from the ice, forces can act on the stones and energy can enter or escape.


 

Image by Photo Mix from Pixabay

Assumptions

A truly isolated system is impossible to create. Isolating a system in terms of mass is fairly straightforward. However, isolating the system with respect to energy and external forces, especially gravity, is far more difficult.  

When we examine the momentum of a system in this section we will make two assumptions so that a useful model can be used to approximate real-world situations.

  1. We will assume that the system is isolated from net external forces to simplify questions.

  2. We will assume that the force of friction acting in a system is negligible, in other words, that is so small that it will not be taken into account in any calculation.

How to Determine if a System is Isolated


To determine if a system is closed just ask yourself if any mass enters or exits. To determine if a system is isolated, however, you must be aware of any external forces that may be acting on the system. Here we will look at some examples. Click to expand the rows.


Two basketballs (the system) are moving vertically in the air.  One of the balls is falling down, while the other has just been thrown upward. The two balls collide as they move.

The system is not isolated.  The external force acting on both balls is the force of gravity.


Two cars (the system) are driving toward one another on smooth pavement and collide head on.

If we assume that the force of friction is negligible, this system can be treated as isolated.

Interestingly, in the vertical plane, the normal force of the road balances the force of gravity, so there is no net external force in the vertical plane

 

Two skaters (the system) are coasting along frictionless ice and are headed straight for each other. They end up crashing into each other.


If we assume that the force of friction is negligible, this system can be treated as isolated.  Again, there is no net external force acting in the vertical plane.