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Chapter 6

Chapter 6. Momentum. This chapter is concerned with inertia and motion. Momentum helps us understand collisions. Momentum and Collisions. Momentum. Momentum = mass X velocity p = mv Momemtum is a vector. Sample Questions.

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Chapter 6

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  1. Chapter 6 Momentum

  2. This chapter is concerned with inertia and motion. Momentum helps us understand collisions. Momentum and Collisions

  3. Momentum • Momentum = mass X velocity • p = mv • Momemtum is a vector

  4. Sample Questions • Which has more momentum, a 1-ton car moving at 100 km/hr or a 2-ton truck moving at 50 km/hr? • Car p = mv = (1 ton)(100km/hr) • Truck p = (2 ton)(50 km/hr)

  5. Large Momentum Examples: • Huge ship moving at a small velocity • High velocity bullet P = mv P = mv

  6. Impulse • Newton’s Second Law can read SF = ma = m(Dv/Dt) = (Dmv)/(Dt) = (Dp/ Dt) Rearranging, Impulse = Dp = FDt

  7. Sample Question • Does a moving object have impulse? Impulse is not a property of the object, but something that it can give or get from an interaction. Notice that it is not motion that gives us an impulse (v) but a change in motion (Dv).

  8. Sample Question • Does a moving object have momentum? Yes, but recall that motion is relative, so the momentum depends on having velocity with respect to the standard of rest.

  9. When Force is Limited • Apply a force for a long time. • Examples: • Follow through on a golf swing. • Pushing a car. FDt

  10. Make it Bounce p1 p2 = -p1 Dp = p2 - p1 = -p1 - p1 = -2p1

  11. Minimize the Force • To minimize force … • Increase Dt • catching a ball • Bungee jumping FDt

  12. Apply a force for a short time. Examples: Boxing Karate Maximize Momentum Change FDt

  13. Conservation of Momentum • If SF = 0, then impulse = Dp = zero, or Momentum is conserved

  14. Demonstrations • Rocket balloon • Cannon • Rocket Scooter

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

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

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

  18. Collisions Air Track Link

  19. Elastic Collisions Conserve Energy and Momentum Before Collision Equal masses Case 1: Case 2: M > M Case 3: M < M

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