Collisions and Momentum 3.1 pp 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!   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 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 v 3 represent? What does m 2 represent? Momentum Before Momentum After

Equations for Collisions  Inelastic Collisions:  The masses stick together after the collision.  What does v 3 represent? What does m 2 represent?  In this case, v 3 represents the speed of both objects after the collision. Momentum Before Momentum After

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

 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