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What is Momentum?

What is Momentum?. An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg ∙m/s m=mass v=velocity Momentum is also a vector (it has direction). How hard is it to stop a moving object?. To stop an object, a force must be applied for a period of time.

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What is Momentum?

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  1. What is Momentum? An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg∙m/s m=mass v=velocity Momentum is also a vector (it has direction)

  2. How hard is it to stop a moving object? To stop an object, a force must be applied for a period of time. This is called Impulse Impulse = FΔt Units: N∙s F = force (N) Δt = time elapsed (s)

  3. How hard is it to stop a moving object? • Using Newton’s 2nd Law we get FΔt= mΔv Which means Impulse = change in momentum

  4. Why does an egg break or not break? • An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? FΔt= mΔv In both cases, m and Δv are the same. If Δt goes up, what happens to F, the force? Right! Force decreases. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time needed to stop the egg.

  5. A Thought Experiment: • Suppose that you were captured by an evil physicist who gave you the following choice: • You must either: • Stand in front of a 1000 kg. truck moving at 1 m/s, or • Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. • …think…

  6. Truck: • Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s • Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules • Meatball: • Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s • Meatball's kinetic energy = 0.5 mv2 = (0.5)(1kg) (1000 m/s)2 = 500 000 Joules

  7. Diagramming: • A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momenta of each player is represented by a momentum vector. Label the magnitude of each momentum vector.

  8. Answer to previous slide question www.physicsclassroom.com

  9. impulse = change in momentum • www.physicsclassroom.com

  10. Bouncing increases impulse explain... • www.physicsclassroom.com

  11. Impulse= change in momentum • www.physicsclassroom.com

  12. Elastic collisions: momentum is redistributed or transferred without loss. m1v1o+ m2v2o= m1 v1f +m2v2f

  13. Inelastic collisions: when two objects become entangled m1v1o+ m2v2o= (m1+m2)vf

  14. M M M M Inelastic Collisions v = 10 v = 0 Before Collision p = mv v’ = 5 After Collision p = 2mv’ mv = 2mv v’ = ½ v

  15. Granny & Little Timmy: calculate the final velocity. • www.physicsclassroom.com

  16. Law of Conservation of Momentum • In the absence of an external force, the momentum of a system does not change. • www.physicsclassroom.com

  17. Truck Collision • In a head-on collision: • Which truck will experience the greatest force? • Which truck will experience the greatest impulse? • Which truck will experience the greatest change in momentum? • Which truck will experience the greatest change in velocity? • Which truck will experience the greatest acceleration? • Which truck would you rather be in during the collision? http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1

  18. http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c2 Ride in the BIG truck! 

  19. Stopping a Truck • Suppose a large truck and a small truck are moving with equal momentum. Which will require more work to stop? Which will require more force to stop in a given distance? http://hyperphysics.phy-astr.gsu.edu/hbase/truckc2.html#c1

  20. More Force to Stop the Little Truck!?? • Since the little truck is moving substantially faster it will have more Kinetic Energy!

  21. Impulse http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html

  22. YIKES! • If the boulder and the boyhave the same momentum,will the boulder crush the boy? • …do not try this at home! http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html

  23. Two Dimensional Collision • The purple ball which is twice as massive as the green ball, is initially moving at 5 m/s. After the collision the purple ball moves at 3 m/s at 20º North of East. What is the velocity of the green ball after the collision? http://turing.kingsu.ca/~map/java/applets/collisions_2D/applethelp/lesson/fig01a.png

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