# Collisions

A person with a large amount of momentum is harder to stop than one with a small amount of momentum. Since a player's mass is constant, players increase their momentum by increasing their speed on the ice. When a collision takes place on the ice, some or all of their momentum is transferred to the other player involved in the collision. This increases the velocity of the opponent while reducing the speed of the attacking player (if they collided head on). During a collision, on smooth surfaces,momentum is conserved.

## Collision Energy Calculator

### http://www.exploratorium.edu/hockey/checking2.html

To find the final velocity, you use the fact that the initial momentum (mass x velocity) of both players must equal the final momentum of the players:

(mass player 1 x velocity player 1) + (mass player 2 x velocity player 2) = combined mass x final velocity

Notice that in the above equation we know all the variables except for the final velocity. We solve for this and get:

final velocity = [(mass player 1 x velocity player 1) + (mass player 2 x velocity playe r2)] combined mass

The energy comes out in a metric unit called a "joule". A joule is not a lot of energy. It's about the amount of energy you'd use to lift an apple to the height of your waist (1 meter).

To find the stoping force, we assumed the collision between the players took about 1/4 of a second. Knowing this, we can look at the change in momentum of either player and use the formula:

force =change-in-momentumtime of impact

According to Newton's third law -- for every action there is an equal and opposite reaction -- each player must experience the same force.