Chapter 5

1. Chapter 5 Momentum Presented by April Senger

2. Momentum • The inertia of motion is called momentum • A truck has more momentum than a car • p = m x v or mass times velocity • What has more momentum, a high velocity bullet or a slow moving train? Presented by April Senger

3. Concept Check • Can you think of a case where a car and a truck with twice the car’s mass would have the same momentum? Presented by April Senger

4. Impulse Changes Momentum • If momentum changes the mass, velocity or both change • Most of the time the mass does not change so the velocity does • If the velocity changes, it is accelerating and accelerations are produced by forces • The greater the force, the greater the velocity, and the greater the momentum Presented by April Senger

5. Increasing Momentum • To increase momentum you can increase the force, the time or both • If I get my car stuck and I begin rocking it the momentum is only changed for a short amount of time • Forward motion will have the greatest momentum if the forces is allowed to act over a longer period of time • I might choose to put a piece of wood under the tire or perhaps put sand around the wheel on ice to that I can increase the force for longer Presented by April Senger

6. Tell Me Why… • Would we want to use a very long cannon? • Should a wrestler relax their muscles before hitting a mat during a throw? • When you jump out of the back of a truck you land with your knees bent? • Use a bungee cord instead of regular rope when jumping off a bridge? • Perform ballet on wooden floors rather than concrete? • Cushion an egg backwards in the egg toss? • Boxers rolls with a punch? Presented by April Senger

7. Contact Time • If you are playing basketball and you are moving your hand forward to receive a pass and jam a finger, why does this result in more damage than if your finger was hit in a still position? • Never move into a punch…Why? • How can a black belt in Karate break a stack of boards or concrete? Presented by April Senger

8. Concept Check • If a boxer rolls with a punch and makes the punch last 3 times as long, by how much is the force reduced? • If the boxer instead moves into the punch and shortens the contact time by half, by how much is the force increased Presented by April Senger

9. Bouncing • If you catch a falling object it will require a certain amount of force to zero it out • It will take even more for to zero it and throw it back in the opposite direction (bouncing) • Any object that bounces has a greater momentum due to this greater force Presented by April Senger

10. Smarter Than The Average Bear • Lester A. Pelton was a gold miner and wasn’t impressed with the force produced by a paddle wheel • The water simply hit the flat paddles and produced moderate force for the wheel • He found a way for water to “bounce” by shaping the paddles in a U shape so that the water poured into the U cups, hit the bottoms and bounced back up • Long story short…He made more $ with his patented Pelton wheel than most gold miners Presented by April Senger

11. Concept Check • How does a karate expert’s force on a pile of concrete bricks compare with the force exerted on their hand? • How will the impulse differ if their hand bounces back when striking the concrete (time stays constant)? Presented by April Senger

12. Net Momentum • Again perspective is important • If you look at a cannon ball and cannon as a system the momentum of the ball leaving is equal to the recoil of the cannon (leaving out gases expelled etc) • Overall the change in momentum is zero or conserved • Notice it isn’t created or destroyed Presented by April Senger

13. Law of Conservation of Momentum • In the absences of an external force, the momentum of a system remains unchanged is called the Law of Conservation of Momentum • If a tennis ball hits a racket, cars collide, or stars explode the net momentum of the system is the same before as it is after • The momentum is conserved Presented by April Senger

14. Concept Check A high-speed bus and a bug have a head on collision. The sudden change of momentum for the bug splatters it all over the windshield. Is the change in momentum of the bus greater, less or the same as the change in momentum of the bug? Presented by April Senger

15. Elastic Collisions The Law of Conservation of Momentum is conserved Objects collide without being deformed or generating heat They bounce perfectly Ex: A billiard ball hits another billiard ball. The first one stops and the other continues with the momentum Inelastic Collisions The Law of Conservation of Momentum is conserved In a perfect collision they would stick together The objects can collide in the same direction such as a fast moving vehicle hits a slow moving car from behind The objects can hit head on Elastic Collisions vs. Inelastic Collisions Presented by April Senger

16. Concept Check • Two gliders have the same mass and move towards each other at the same speed. They experience an elastic collision. What would their movement look like after the collision? • What if they had an inelastic collision? Presented by April Senger

17. In A Perfect World… • Perfect elastic collisions are rare • We can try to simulate them with an air track • It works a lot like a air hockey table but carts slide back and forth on a track • At an atomic level, collisions cause heat • Bouncing a ball illustrates how the heat transferred to the floor decreases the height a ball can bounce to Presented by April Senger