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Now that we learned a great deal with sliding friction, we will look at momentum. This is an important aspect of physics because momentum is the key to success in hockey. We will later examine the shooting of a puck since it is an application of momentum.

Momentum

A simple definition of momentum is similar to what Newton states: momentum is a "quantity of motion". Likewise, the magnitude of the momentum, p, at a moment is equal to the product of the mass (m) times the instantaneous velocity (v).


P=mv

The SI units for momentum is kg*m/s

Since velocity is a vector, momentum is a vector (definite magnitude and direction).

Momentum is a crucial element for hockey players. A game of hockey consists of many body-checks and fights. In a body check , the player with the greater momentum will the defeat the opponent. Also, heavy player that is moving slow may have less momentum compared to a lighter player who is skating faster.

The Relation to Force

A force is required to change the value or the direction of motion. Newton states that the rate of change of momentum of a body is proportional to the net force applied to it.
Given the equation, F= delta p/delta t , F is the net force applied to an object that is equivalent to a change in momentum at a time interval. (Physics Encyclopedia, "Momentum", 1998)

Impulse

delta p= F * delta t

An impulse is the product of the force and the time which the force act. Also, the total change in momentum is equal to the impulse. The impulse can be accomplished by a large force acting over a short time, or by a small force
acting over a long time. This concept is important when we deal with shooting.

Conservation of Momentum

In the interaction of two bodies, one exerts a force on the other and momentum of each body changes. According to Newton's third law of motion, the two impulses in any time interval are both equal and opposite.
This principle can be easier to understand by defining the total momentum of the system as the sum of the separate bodies. When two bodies interact only with each other, their total momentum is constant.
When there is external forces are absent or the resultant of the external forces are zero, the total momentum of the system is constant, in regards to magnitude and direction. The is a statement of the "Principle of Conservation of Linear Momentum":

When no resultant external force acts on a system, the total momentum of the system remains constant in magnitude and direction.

The Two Types of Collisions

Elastic- A collision in which the total kinetic energy is constant (same with momentum).
ie. Bumping of billiard balls


Inelastic- A collision in which the total kinetic energy is not constant ( but constant momentum).
Ie. Entanglement of smashing cars


Most collisions in real life fall in the category between elastic and inelastic , since perfectly elastic and inelastic cases are rare due to all sorts of factors (friction, deformation, and thermal heat).