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Physics Chapter 6

Physics Chapter 6. Impulse and Momentum. Momentum. Momentum depends on… Mass Velocity Momentum = Mass x Velocity The property of momentum… Variable: p Unit: Scalar or Vector ? Equation: p = mv. Example #1.

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Physics Chapter 6

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  1. Physics Chapter 6 Impulse and Momentum

  2. Momentum • Momentum depends on… • Mass • Velocity • Momentum = Mass x Velocity • The property of momentum… • Variable: p • Unit: • Scalar or Vector? • Equation: p = mv

  3. Example#1 Which has more momentum, a 1-ton car moving at 100 km/h or a 2-ton truck moving at 50 km/h?

  4. Example #2 How fast would a 18 kg kid on a tricycle have to go in order to have the same momentum as a 55 kg car traveling at 25 m/s?

  5. F Impulse • Momentum can change. Most often, the mass of an object remains the same, while the velocity changes. • Dp = mDv • Dv  acceleration a = Dv/t  Dv=at • Dp = mDv becomes Dp = m x a x t • Dp = Force x time = Ft • This is called Impulse • I = Ft

  6. Example #3 If a baseball bat applies a 22-N force to a baseball for 0.13 s, what is the impulse experienced by the ball?

  7. Impulse & Momentum • Impulse equals a change in momentum • I = Dp • Variations Ft = mDv Ft = mvf – mvi I = mDv I = mvf – mvi Ft = Dp

  8. Example #4 What is the impulse needed to stop a 10-kg bowling ball moving at 6 m/s?

  9. Example #5 A car crashes into a wall at 25 m/s and is brought to a rest in 0.1 s. Calculate the average force exerted on a 75-kg test dummy by the seat belt.

  10. Impulse & Momentum Increasing Momentum mDv = Ft Increase the time to increase the velocity • Examples: • Follow through in sports • Basketball shoot • Baseball hit • Soccer kick • Golf swing • Long-range cannons have long barrels

  11. Impulse & Momentum Decreasing Force mDv = Ft Increase the time to decrease the Force • Examples: • Catching • Egg toss • Water balloon • Football • Cars: Crumple Zones & Air Bags • Roll with the punches • Mighty Ducks • Jumping and landing (sprung floors) • Running Shoes • Any padding • Bungee Jumping

  12. Problems • Review Questions 1-11, 13 • Exercises 1-4, 6, 9, 10 • Problems 1, 2, 5

  13. Conservation of Momentum • Newton’s Second Law… • If we want an object to accelerate, we must apply a force. • Impulse and Momentum… • If we want a change in momentum, we must apply an impulse. • In both cases, the force or impulse must be exerted on the object or any system of objects by something external.

  14. Conservation of Momentum • If we define the system to include all the objects we want to study…  all forces become internal;  there can be no impulse;  no impulse means no change in momentum! • When a physical quantity remains unchanged during a process, that quantity is said to be… CONSERVED.

  15. Collisions • Momentum is conserved  Momentum before = Momentum after • Three types of collisions… • Elastic objects rebound off each other without lasting deformation. • Inelastic  objects are deformed by the collision; think car crashes. • Perfectly inelastic  both objects stick together

  16. v = 10 m/s v = 0 v = ? Example #1a The blue car is moving at 10 m/s, with a mass m. The orange car, also mass m, is at rest. If the freight cars are coupled together by the collision, what is their combined velocity?

  17. v = 10 m/s v = ? v = 0 v = 0 Example #1b If the freight cars in the previous example do not couple and experience an elastic collision, what is the velocity of the orange car?

  18. v = ? v = 10 m/s v = 0 Example #2 Two trucks with equal masses, m, experience a head on collision. After collision, the coupled wreck remains at the point of impact with zero momentum. If the green truck was moving at 10 m/s, how fast was the red truck moving?

  19. Example #3 A 3 kg object traveling at 5 m/sec collides head-on with a 2 kg object that is traveling at 2 m/sec in the opposite direction. What is the velocity of the second object after the collision if the first object is moving at 1 m/sec?

  20. Problems • Review Questions 18-23 • Exercises 13, 21 • Problems 6-10

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