Unit 7.2 Conservation of Momentum

Teacher: Dr. Van Der Sluys

Objectives

• Collisions

• Law of Conservation of Momentum

• Simple Collision Problems– Elastic– Inelastic

Momentum

For an individual object

p = mV

For a system involving multiple objects

p = mnvn

Where mnvn are the momentums of each of the individual objects in the system.

Collisions

• Elastic collisions between objects result in rebounds, so that the total kinetic energy of the system remains the same.

• Inelastic collisions result when the objects in the system stick together and there is a conversion of some kinetic energy into other forms of energy.

Law of Conservation of Momentum

• For any two body system the total momentum before and after a collision must be equal.

m1v1 + m2v2 = m1v1’ + m2v2’

• The law of Conservation of momentum states that the total momentum of an isolated system is always constant.

Conservation of Momentum - Rifle recoil

What is the recoil velocity of a 6.0 kg shot gun that shoots a 0.150 kg deer slug at a speed of 120 m/s?

Hint: Draw a vector diagram representing both the velocity and momentum of both the bullet and the gun before and after the bullet is fired.

Conservation of Momentum - Simple Inelastic Example

An empty 10,000 kg railroad car traveling at a speed of 24.0 m/s strikes an identical railroad car that is stationary. If the couplings cause the cars to lock together as a result of the collision, what is their velocity after the collision?

Conservation of Momentum - Simple Inelastic Example

An empty 10,000 kg railroad car traveling at a speed of 24.0 m/s strikes loaded railroad car having a mass of 20,000 kg that is stationary. If the couplings cause the cars to lock together as a result of the collision, what is their velocity after the collision?

Conservation of Momentum - Simple Inelastic Example

An empty 10,000 kg railroad car traveling at a speed of 24.0 m/s strikes loaded railroad car having a mass of 20,000 kg that is moving with a velocity of -12.0 m/s. If the couplings cause the cars to lock together as a result of the collision, what is their velocity after the collision?

Simple Systems Involving Elastic Collisions

• http://www.hazelwood.k12.mo.us/~grichert/explore/dswmedia/airtrack.htm

• Try some of the following and predict what will happen after the elastic collision.– m1 = m2, v1 = 5.0 m/s and v2 = 0 m/s (Can you

predict? How do you know?)– m1 = 1.0 kg and m2 = 2.0 kg, v1 = 5.0 m/s and v2 = 0

m/s– m1 = m2, v1 = 5.0 m/s and v2 = -5.0 m/s– Try others of your choice (Can you predict? If not,

why not?)

A Really Complicated System Involving Elastic Collisions in Two

Dimensions

http://en.wikipedia.org/wiki/Elastic_collision