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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 (J) J = 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 time (Δt) increases, what happens to the amount of force (F)? Answer: 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; therefore, less force is applied to 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. Using vectors to understand • A 120 kg lineman moving west at 2 m/s tackles an 80 kg fullback moving east at 8 m/s. • After the collision, both players move east at 2 m/s. • Draw vector diagrams for the before AND after situations; draw two diagrams.

  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 vi= 10m/s vi= 0 (at rest) Before Collision p = mv vf= 5m/s After Collision p = 2mv mredvred +mbluevblue= m(red+blue)vf

  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. 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

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

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

  20. 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

  21. 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

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

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