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Chapter 6

Chapter 6. Momentum. Units - kg m/s or sl ft/s. 1. MOMENTUM. Momentum - inertia in motion Momentum = mass times velocity. Units - N s or lb s. 2. IMPULSE. Collisions involve forces (there is a D v ). Impulse = force times time. 3. IMPULSE CHANGES MOMENTUM. Impulse = change in momentum.

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Chapter 6

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  1. Chapter 6 Momentum

  2. Units - kg m/s or sl ft/s 1. MOMENTUM • Momentum - inertia in motion • Momentum = mass times velocity

  3. Units - N s or lb s 2. IMPULSE • Collisions involve forces (there is a Dv). • Impulse = force times time.

  4. 3. IMPULSE CHANGES MOMENTUM Impulse = change in momentum

  5. Case 1Increasing Momentum • Follow through • Examples: • Long Cannons • Driving a golf ball • Can you think of others?

  6. Video Clip Tennis racquet and ball

  7. Case 2Decreasing Momentum over a Long Time Examples: Rolling with the Punch Bungee Jumping Can you think of others? Warning – May be dangerous

  8. Case 3Decreasing Momentum over a Short Time Examples: Boxing (leaning into punch) Head-on collisions Can you think of others?

  9. 4. BOUNCING There is a greater impulse with bouncing. Example: Pelton Wheel Demo – Impulse Pendulum

  10. Consider a hard ball and a clay ball that have +10 units of momentum each just before hitting a wall. • The clay ball sticks to the wall and the hard ball bounces off with -5 units of momentum. • Which delivered the most “punch” to the wall?

  11. Initial momentum of the clay ball is 10. Final momentum of clay ball is 0. The change is 0 - 10 = - 10. It received - 10 impulse so it applied + 10 to the wall.

  12. Initial momentum of the hard ball is 10. Final momentum of hard ball is - 5. The change is - 5 - 10 = - 15. It received - 15 impulse so it applied + 15 to the wall.

  13. 5. CONSERVATION OF MOMENTUM Example: Rifle and bullet Demo - Rocket balloons (several) Demo - Clackers Video - Cannon recoil Video - Rocket scooter

  14. Consider two objects, 1 and 2, and assume that no external forces are acting on the system composed of these two particles. • Impulse applied to object 1 • Impulse applied to object 2 • Apply Newton’s Third Law • Total impulse • applied • to system • or

  15. Internal forces cannot cause a change in momentum of the system. • For conservation of momentum, the external forces must be zero.

  16. Chapter 6 Review Questions

  17. The product of mass times velocity is most appropriately called (a) impulse (b) change in momentum (c) momentum (d) change in impulse

  18. You jump off a table. When you land on the floor you bend your knees during the landing in order to (a) make smaller the impulse applied to you by the floor (b) make smaller the force applied to you by the floor (c) both (a) and (b)

  19. An egg dropped on carpet has a better chance of surviving than an egg dropped on concrete. The reason why is because on carpet the time of impact is longer than for concrete and thus the force applied to the egg will be smaller. (a) True (b) False

  20. 6. COLLISIONS • Collisions involve forces internal to colliding bodies. • Elastic collisions - conserve energy and momentum • Inelastic collisions - conserve momentum • Totally inelastic collisions - conserve momentum and objects stick together

  21. Demos and Videos • Demo – Air track collisions (momentum & energy) • Demo - Momentum balls (momentum & energy) • Demo - Hovering disks (momentum & energy) • Demo - Small ball/large ball drop • Demo - Funny Balls • Video - Two Colliding Autos (momentum) Terms in parentheses represent what is conserved.

  22. Simple Examples of Head-On Collisions (Energy and Momentum are Both Conserved) Collision between two objects of the same mass. One mass is at rest. Collision between two objects. One at rest initially has twice the mass. Collision between two objects. One not at rest initially has twice the mass.

  23. Head-On Totally Inelastic Collision Example • Let the mass of the truck be 20 times the mass of the car. • Using conservation of momentum, we get

  24. initial momentum of system = final momentum of system

  25. Remember that the car and the truck exert equal but oppositely directed forces upon each other. • What about the drivers? • The truck driver undergoes the same acceleration as the truck, that is

  26. The car driver undergoes the same acceleration as the car, that is The ratio of the magnitudes of these two accelerations is

  27. Remember to use Newton’s Second Law to see the forces involved. • For the truck driver his mass times his acceleration gives For the car driver his mass times his greater acceleration gives

  28. , big trucks that is. • Your danger is of the order of twenty times greater than that of the truck driver. TRUCKS • Don’t mess with T

  29. 7. More Complicated Collisions Vector Addition of Momentum

  30. Example of Non-Head-On Collisions (Energy and Momentum are Both Conserved) Collision between two objects of the same mass. One mass is at rest. If you vector add the total momentum after collision, you get the total momentum before collision.

  31. Examples: Colliding cars Exploding bombs • Video - Collisions in 2-D

  32. Chapter 6 Review Questions

  33. In which type of collision is energy conserved? (a) elastic (b) inelastic (c) totally inelastic (d) All of the above (e) None of the above

  34. A Mack truck and a Volkswagen have a collision head-on. Which driver experiences the greater force? (a) Mack truck driver (b) Volkswagen driver (c) both experience the same force

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