Physics 11

Energy 8 – Conservation of Momentum

Mr. Jean

The plan:

• Video clip of the day

• Conservation of Momentum

Conservation of Momentum:

All momentum in a system before any interaction must be equal to all momentum afterwards.

Law of Conservation of Momentum:

• Now suppose that a medicine ball is thrown to a clown who is at rest upon the ice; the clown catches the medicine ball and glides together with the ball across the ice. The momentum of the medicine ball is 80 kg*m/s before the collision. The momentum of the clown is 0 m/s before the collision. The total momentum of the system before the collision is 80 kg*m/s. Therefore, the total momentum of the system after the collision must also be 80 kg*m/s. The clown and the medicine ball move together as a single unit after the collision with a combined momentum of 80 kg*m/s. Momentum is conserved in the collision.

One Dimensional Collisions:

• In a one dimensional collision both the magnitude and the direction of the momentum must be conserved.

• For complex momentum situations break all momentums into components and then sum the components. This too will conserve momentum.

Collisions:

• Collisions can be classified according to the energy interaction that takes place:

• Elastic collision kinetic energy is conserved

• Inelastic collision kinetic energy is not conserved

• Perfectly inelastic collision objects stick together and have the same velocity.

Example:

• An old lady driving a 2.5x103 kg H2 Hummer drives into the back of Ben’s 1.0x103 kg Subaru WRX STI. The two cars stick together. What is their velocity after the collision if the old lady was originally travelling at 8.0m/s?

Example #2:

• The cue ball collides with the ‘8’ ball. The cue ball has twice as much mass as the ‘8’ ball. The objects do not stay attached. – What is the velocity of the ‘8’ ball?– Is this collision realistic? (Why or why not?)

Example #3: Cannon Recoil • A 2000kg cannon contains a 100kg

armour piercing shell. The cannon fires the projectile horizontally with a velocity of 1000m/s. – What is the velocity of the cannon after the

shot?

• Before: After:

“Loose Cannon”:

• An unpredictable person or thing, liable to cause damage if not kept in check by others.

• Also a place to eat in Halifax on Argyle Street.

Rifle Recoil:

• .50 Cal Rifle

• “Surprising Recoil”