Optional Unit V: Applications of Kinematics and Dynamics
A. Momentum

2. The Law of Conservation of Momentum

Key Concepts

The total momentum of an isolated system does not change. (Law of Conservation of Momentum) Another way of stating this is that the initial momentum of an isolated system is equal to its final momentum.

An isolated system is one in which no net external force acts on the system.

Momentum is conserved regardless of whether the interactions in the system occur in more than one dimension.

(The ' symbol is used to indicate that the quantity is being considered after an interaction has taken place.)

For two objects of unequal mass colliding:

or

The centre of mass of an isolated system is a special point, obeying Newton's First Law. From that frame of reference, the total momentum of the system being observed would be zero.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: isolated system, centre of mass.

2. Recognize that the Law of Conservation of Momentum is one of the most fundamental principles in physics.

3. Recognize that momentum is conserved in an isolated system in one or more dimensions .

4. Apply mathematical expressions of the Law of Conservation of Momentum to problem solving .

5. Recognize the significance of the centre of mass of an isolated system.

Teaching Suggestions, Activities and Demonstrations

1. Analyze stroboscopic photographs to verify the conservation of momentum in an isolated system, such as a billiard ball collision.