
Physics Tutorial

Momentum and Its Conservation

Lesson 1

Lesson 2


	The Law of Action-Reaction (Revisited)

	Momentum Conservation Principle

	Isolated Systems

	Using Equations as a "Recipe" for Algebraic Problem-Solving

	Using Equations as a Guide to Thinking

Lesson 2: The Law of Momentum Conservation

Isolated Systems

The previous part of Lesson 2 focused on the Law of Conservation of Momentum. It was stated that ...

For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.

Total system momentum is conserved for collisions occurring in isolated systems. But what makes a system of objects an isolated system? And is momentum conserved if the system is not isolated? This is the focus of this part of Lesson 2.

collision between billiard balls A system is a collection of two or more objects. An isolated system is a system which is free from the influence of a net external force. There are two criteria for the presence of a net external force; it must be...

a force which originates from a source other than the two objects of the system

a force that is not balanced by other forces.

Consider the collision of two balls on the billiards table. The collision occurs in an isolated system as long as friction is small enough that its influence upon the momentum of the billiard balls can be neglected. If so, then the only unbalanced forces acting upon the two balls are the contact forces which they apply to one another. These two forces are considered internal forces since they result from a source within the system - that source being the contact of the two balls. For such a collision, total system momentum is conserved.

Read the following descriptions of a collision and evaluate whether or not the collision occurs in an isolated system. If it is not an isolated system, then identify the net external force.

Collision Description
Isolated System? Yes or No
If No, then the external force is...

1.
Two cars collide on a gravel roadway on which frictional forces are large.

2.
Hans Full is doing the annual vacuuming. Hans is pushing the Hoover vacuum cleaner across the living room carpet.

3.
Two air track gliders collide on a friction-free air track.

If a system is not isolated, then the total system momentum is not conserved. To understand this concept, we will return to our original analogy. Suppose Jack and Jill (each with $100 in their pockets) undergo a financial interaction in which Jack hands Jill $50 for the purchase of some goods. If Jack and Jill were isolated from the influence of the rest of the world, then Jack would end up with $50 and Jill would end up with $150. The total money in the system would be $200 both before and after the transaction; total system money would be conserved. If however, a third influence enters from outside of the system to take away or (more fortunately) to add money to the system, then total system momentum would not be conserved. If a thief interfered with his filthy hands so as to steal $20, then perhaps Jack would finish with $40 and Jill would finish with $140. In the case of a non-isolated system, the total momentum is not conserved.

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