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##### Momentum

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**Can you name 5 safety features found in an automobile?**• Seat belts • Air bags • Bumpers • Padded consoles, collapsible steering wheels, padded interior • Crumple zones**Momentum, Impulse, and Auto Safety**The concepts of momentum, conservation of momentum, and impulse explain a lot of the science behind car crashes and the safety devices that have been designed because of those crashes.**momentum & impulse**Examples of impulses being applied on everyday objects**Momentum**• What is momentum? • Anything that is moving has momentum • Depends on what?**Momentum**• Momentum = mass * velocity • In physics, the symbol for momentum is “p” sssooo… • p = m * v measured in kg·m/s Which has more momentum, a supertanker tied to a dock or a raindrop falling?**How to increase momentum?**Which has more momentum… • Increase either Mass or velocity • A train standing still • A little car moving at 15 m/h**Can the train and the car be made to have the same momentum?**• Yes • No We can either stop the car…or we can speed up the train**Momentum**• momentum = mass x velocity p = mv • Which has more momentum: a 300lb football player moving at 5m/s or a 200lb quarterback moving at 10m/s?**Answer**• Momentum of the 300lb player is 300lbs x 5m/s = 1500lb-m/s • Momentum of the quarterback is 200lbs x 10m/s = 2000lb-m/s • The quarterback has a greater momentum!**Practice**• A 100 kg (220 lbs)football player runs with a velocity of 9 m/s towards the visitors end zone. What is his momentum? M=100 kg V=9 m/s Known: Unknown: P=? P=mv P=100 kg X 9 m/s Plug and Chug P=900 kg * m/s**Practice**• A 0.5 kg volleyball is spiked at a rate of 10 m/s towards the ground. What is the momentum of the volleyball? M=0.5 kg V=10 m/s Given: Unknown: P=? P=mv P=0.5 kg X 10 m/s Plug and Chug P=5 kg * m/s**Momentum**• Determine the momentum of a ... • 60-kg halfback moving eastward at 9 m/s. • 60 x 9 = 540 kg·m/s east • 1000-kg car moving northward at 20 m/s. • 1000 x 20 = 20,000 kg·m/s • 40-kg freshman moving southward at 2 m/s. • 40 x 2 = 80 kg·m/s**Impulse Changes Momentum**• The impulse exerted on an object depends on: • the force acting on the object • the time the force acts Impulse = change in momentum (p) Impulse = Force x time Therefore… Ft = Dp**Impulse Changes Momentum**Increasing Momentum Apply the greatest force possible for the longest time possible. This creates the greatest impulse possible. Example: Follow through in sports**Impulse Changes Momentum**Decreasing Momentum • Apply a small force over a long time. A longer contact time reduces force. Example:**Impulse Changes Momentum**Decreasing Momentum • Apply a large force over a small time. A shorter contact time increases force. Example:**Remember: Ft = Dp**How do the safety features of a car protect you in a collision? • In an accident, the Dp stays the same, but you can change F and t of impact. • When you increase the time that the collision lasts, the force of impact decreases. Ft • All safety features in a car are designed to reduce the force by increasing the time you are in contact with that force.**Other examples that decrease the force by increasing the**time • Air bags • Catching a baseball with a glove • Fighting with padded gloves v. bare fists • Rolling with a punch • Rocket propulsion • Packing materials**Real Life Applications..**• Sports… • Follow-through! • Gymnastics Mats • Cars… • Airbags! • Before airbags…. • Bumpers • Jumping… • Knees • Shoes**Baseball**Q: Why batters swing when they hit a ball? A: To increase the time of contact Catching an Egg Q: An egg is thrown at you. How can you catch it without breaking it?**Momentum Impulse**• An egg (0.1 kg) is flying through the air at 20. M/s. • How are we going to stop it?**BOUNCING**• The impulse required to bring an object to a stop and then to “throw it back again” is greater than the impulse required to merely bring the object to a stop.**Momentum and Impulse**A tennis ball bouncing off the floor. There is a rapid change in the direction of the velocity when the ball hits the floor.**Bouncing Tennis ball**The floor delivers the impulse to the tennis ball.**The Law of Conservation on Momentum**• The total momentum of two objects before a collision is equal to the total momentum of the two objects after the collision.**What happens in a collision?**• Collisions produce a change in momentum (p=mv) on an individual object. • For example, if you are traveling at 60 mi/h (27 m/s) in a 1000 kg car, then you have a momentum of 27,000 kg m/s. If you collide with a brick wall, your momentum will be reduced to 0 kg m/s. [Dp=27,000 kg m/s] http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/newtlaws/cci.html**Collisions**Two types of collisions involving momentum • Elastic-objects bounce off each other • Bowling balls and pins • Inelastic-objects stick together • Football Tackle • The two will continue in the direction of the player with the greatest momentum**Elastic Collisions**In an elastic collision, the kinetic energy of the system is unchanged by the collision Example: In billiards, collisions between balls are almost completely elastic http://www.billiard-tables-cues.com/images/billiard%20balls.jpg**Inelastic Collisions**An inelastic collision results in a decrease in a system’s total kinetic energy. Kinetic energy is transformed into other types of energy e.g. thermal Example: When football players hit each other, collisions are inelastic**Happy/Sad Ball Demonstration**• Happy ball is made of neoprene rubber. It bounces when it hits the floor—energy is conserved so this is an ELASTIC collision • Sad ball is made of norbornene. It does not bounce when it hits the floor because most of the energy is dissipated in the collision. This is an INELASTIC collision.**Before the collision**P1 Momentum of running back is 100kg x 5m/s = 500 kg m/sMomentum of linebacker is 75 kg x (-4 m/s) = -300 kg m/s What is the total momentum before the collision? • 500 kg m/s • 300 kg m/s • 200 kg m/s**After the collision**Momentum of the two players before and after the collision is the same (200 kg m/s) Momentum after (P2) must be 200 kg m/s P2 = 1.14 m/s x 175kg = 200kg m/s = P1**Momentum Practice**2 cars are heading east, car A is traveling 30mi/hr, car B is traveling 60mi/hr. Each car weighs 2000lbs. • What is the momentum of car A? • What is the momentum of car B? • If car B crashes into car A, what is the total momentum?**Answer**p=mv Car X’s momentum = 30mi/hr x 2000lbs pX = 60,000 mi-lbs/hr east Car Y’s momentum = 60mi/hr x 2000lbs pY = 120,000 mi-lbs/hr west Total momentum = pY - pX = 120,000 - 60,000 = 60,000 mi-lbs/hr west**non-violent collisions**• Two stationary ice skaters push off • both skaters exert forces on each other • however, the smaller skater acquires a larger speed than the larger skater.**7.3 Recoil**Q: Why does a shot gun slam against your shoulder when fired?**Two Particle Collisions**• Elastic Collisions p1B + p2B = p1A + p2A m1v1B + m2v2B = m1v1A + m2v2A • Inelasic collisions p1B + p2B = p1+2A m1v1B + m2v2B = m1+2v1+2A