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Momentum

Momentum. Momentum. I nertia in motion momentum (p) is equal to mass x velocity units for momentum: kg· m/s. Which will have more momentum, a semi moving at 70 km/h or a Prius moving at 70 km/h? Why ?. Will a large truck ALWAYS have more momentum than a Pruis ?

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Momentum

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  1. Momentum

  2. Momentum • Inertia in motion • momentum (p) is equal to mass x velocity • units for momentum: kg· m/s

  3. Which will have more momentum, a semi moving at 70 km/h or a Priusmoving at 70 km/h?Why? • Will a large truck ALWAYS have more momentum than a Pruis? • NO!!! If the truck is at rest, it has no momentum.

  4. Problem Solving Practice! Momentum practice: • A child on a sled is moving at 5 m/s. The mass of the child and the sled is 90 kg. What is the momentum? Given Equation Substitution Answer 90kg p = mv p = (90kg)(5m/s) p = 450 kg·m/s 5m/s

  5. Journal: Your Turn A 70 kg skateboarder launches off of a ramp at a velocity of 14 m/s. What is the skateboarder’s momentum at that instant? • p = (70kg)(14m/s) = 980 kg·m/s

  6. A 725 kg car is moving at 115km/h. What is it’s momentum in kgm/s? (watch your units) 115 km/h (1000 m/km)( 1 hr/ 60 min) (1 min/ 60 sec) = 31.9 m/s • p = mv • p = (725kg) (31.9 m/s) = 23,172.5 kg·m/s

  7. Journal: When does momentum of an object change???? Use the equation for momentum as your reference!!!!

  8. Momentum of an object changes if: • mass changes • velocity changes • or both change!! When do velocity changes occur???

  9. Impulse: how long a force acts on an object • impulse = FΔt Change in momentum is called impulse. or Ft = Δmv Ft = Δp

  10. Know that a= F/m a= v/t Journal: Derive the impulse equation, Ft = Δmv, from the two equations for acceleration. Show your work.

  11. Increasing Momentum of an object: apply a large force for as long as possible Ft = Δp

  12. Decrease momentum: • Extending impact time reduces the force of the impact. • Example: catching a baseball and pulling your arm back to slow the catch

  13. Bouncing • Impulses are great because the impulse required to both “stop” the object and then “throw it back again”

  14. Law of Conservation of Momentum • In the absence of an external force, the momentum of a system remains unchanged. • initial momentum = final momentum

  15. Law of Conservation of Momentum = Example: Two cars colliding • The net momentum of the system is the combined momentum of the cars colliding, so before and after the collision the total momentum of the system is the same. =

  16. Collisions: • Elastic • Inelastic

  17. Elastic: when objects collide without being deformed or generating heat • bounce perfectly • Momentum is reversed • Kinetic Energy is conserved =

  18. Example: A 300 kg bumper car travelling at 10 m/s collides with a 400 kg bumper car that is stopped. After the elastic collision, the 300 kg car is travelling in the opposite direction at 1.43 m/s. What is the resulting velocity of the 400 kg car after this collision? [ (m1)(v1) + (m2)(v2) ] initial= [ (m1)(v1) + (m2)(v2) ] final [ (300kg)(10 m/s) + (400kg)(0 m/s)] = [ (300kg)(-1.43 m/s) + (400 kg)v2] • 3000 = - 429 + 400v2 • 3429 = 400v2 • 3429/ 400 = v2 • 8.57=v2

  19. Inelastic : when objects collide and are deformed • Kinetic Energy is NOT conserved, rather it is transferred into thermal or potential energy. =

  20. Practice Problem • A car weighing 200 kg and travelling at 25 m/s crashes and sticks to a 1500 kg car that was travelling at 20 m/s. What is the final velocity of the joined vehicles? = • (2000)(25) + (1500)(20) = ( 2000+1500) Vf • Vf= 22.86

  21. For both Elastic and Inelastic collisions, TOTAL ENERGY IS CONSERVED!!!!

  22. Practice Problem Solving

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