Collisions and Momentum3.1 pp. 73-77

Mr. Richter

Agenda

Warm-Up More about Science Fair Topics Introduction to Collisions Notes:

Elastic vs. Inelastic Collisions Momentum Conservation in Collisions Forces in Collisions and Impulse

Objectives: We Will Be Able To… Distinguish between elastic and inelastic collisions. Use momentum conservation to solve collision

problems. Describe the difference in forces in elastic and inelastic

collisions.

Warm-Up:

Two cars traveling the same speed crash into a barrier. Car A crumples the bumper and hood and remains stuck to the barrier. Car B bounces back off of the barrier and looks relatively unharmed. Which car experienced a greater force, assuming the accidents took place in the same amount of time?

On a separate sheet of paper, write your name. Then write 2-3 complete sentences answering the question. You have 5 minutes.

Science Fair Topics (10 minutes)

Still don’t know what to do for science fair? Watch this video! http://archimedesinitiative.org/choosingadventure.html

Need help brainstorming? Write down an activity or hobby you like to do in your free

time. Write three questions you have about this activity or

hobby. Brainstorm how you might use a science fair project to

help answer one or all of these questions.

Collisions

Collisions

Collisions can be categorized into two types: elastic inelastic

Elastic collisions are when objects bounce off of each other. (Elastics are like rubber bands, and

rubber bounces) Car B

Inelastic collisions are when objects stick together after the crash. Car A

Elastic Collisions

In perfectly elastic collisions objects: Bounce off each other No loss of energy due to speed

(kinetic energy) No change of shape.

In real life, there are almost no perfectly elastic collisions. Almost always, some energy is

lost to sound or heat in a collision.

Inelastic Collisions

In inelastic collisions objects: Stay stuck together Speed energy is lost to sound

and heat energy Objects are deformed (shape

is changed.

In real life, most collisions are a combination of elastic and inelastic collisions.

Warm-Up

Name one difference between elastic and inelastic collisions. Then give an example of an elastic collision and an example of an inelastic collision.

On a separate sheet of half sheet of paper. 5 minutes.

Momentum Conservation in Collisions

As long as there are no outside forces (like friction) momentum is conserved in both elastic and inelastic collisions.

This makes it possible to determine the motion of objects before or after colliding.

The equations come from adding the momentum of the objects before the collision and setting it equal to the momentum of objects after the collision. (No loss of total momentum!)

Equations for Collisions

Elastic Collisions:

The masses stay separate after the collision.

What does v3 represent? What does m2 represent?

MomentumBefore

MomentumAfter

Equations for Collisions

Inelastic Collisions:

The masses stick together after the collision.

What does v3 represent? What does m2 represent?

In this case, v3 represents the speed of both objects after the collision.

MomentumBefore

MomentumAfter

Solving Collision Problems

1. Decide whether the collision is elastic or inelastic, and choose the appropriate equation.

2. Assign values for different masses and velocities.

3. Solve the equation and check for reasonableness.

Practice Problem

An 8000-kg train car traveling 10 m/s collides with a 2000-kg that is initially at rest. After the collision, the cars stick together. What is their combined speed after the collision?

Forces in Collisions

Forces in Collisions

In a collision, the greater the change in velocity, the greater the force that was applied.

Think of our cars from the warm up. Car A comes to a stop (inelastic), but Car B stops and then

bounces backward (elastic)! Car B has a greater change in velocity, so it experiences a

greater force. This is why cars are designed to crumple. Elastic Collisions = Bouncing = More Force Inelastic Collisions = Stopping = Less Force

Wrap-Up: Did we meet our objectives?

Distinguish between elastic and inelastic collisions. Use momentum conservation to solve collision

problems. Describe the difference in forces in elastic and inelastic

collisions.

Homework

p.77 #1-4 Due Thursday