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Physics 102: Chapter 6 - Momentum

Physics 102: Chapter 6 - Momentum. Momentum and Energy views of force Impulse-Momentum Theorem What does conservation mean Conservation of Momentum. Impulse. The force of the foot on the ball is an impulsive force. Slide 9-8. Momentum.

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Physics 102: Chapter 6 - Momentum

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  1. Physics 102: Chapter 6 - Momentum Momentum and Energy views of force Impulse-Momentum Theorem What does conservation mean Conservation of Momentum

  2. Impulse The force of the foot on the ball is an impulsive force. Slide 9-8

  3. Momentum Momentum is the product of an object’s mass and its velocity:   p = mv Slide 9-10

  4. The Impulse-Momentum Theorem Impulse causes a change in momentum:     J =pf - pi = ∆p Slide 9-11

  5. Slide 9-15

  6. Example A 500 kg rocket sled is coasting at 20 m/s. It then turns on its rocket engines for 5.0 s, with a thrust of 1000 N. What is its final speed? Slide 9-17

  7. Starting and Stopping A car with a mass of 1 metric ton speeds up to highway speed from rest on a strait section of Central Blvd. A little while later, the car comes to a stop as it approaches a red light. Part 1 - Determine the net impulse and average net force on the car as it goes from rest to highway speed. Part 2 - Determine the net impulse and average net force on the car as it goes from highway speed to a complete stop.

  8. Impulses and Car Crashes • Consider a car going at highway speeds colliding in a front-end collision with a brick wall. Compare the impulse needed to bring the passenger in the front seat to a stop if they are stopped by each of the following: • Their seatbelt • The dashboard • An airbag 2. Rank the force being applied for each case.

  9. Impulse Effects of Time and Force Large time => small force Small time => Large force

  10. Impulse Effects of Time and Force • Pulling your hand back while you catch a ball • Bending your knees and rolling when you fall in Self defense class / Sky-Diving • Falling on a wooden floor is safer than falling on a cement floor • Railroad car couplings are loose => slow to accelerate or stop • Movies (Nail on head / hammer on hand/Inertia Balls)(Table top pull movie)Table top friction pull

  11. Shut the Door You are sittingon your bed in your dorm room, and suddenly you hear the voice of your ex coming down the hall. You really want to avoid any contact (you broke things off a week ago), and so you want to shut the door. But you don't have time to get up and shut it and act like it wasn't on purpose. You need something fast. Sitting beside you, you happen to have a super ball and a ball of clay that you fidget with when you're studying on your bed. What do you do? Explain your answer and show why you chose one and not the other.. (Demonstration movie =>http://groups.physics.umn.edu/demo/collisionframe.html)

  12. Pelton Wheel Enriching yourself with Physics During the California Gold Rush, Lester Pelton designed a water wheel that caused the water to make a U-turn, i.e. causing the water to bounce off the paddle. He made a lot of money on this invention, more than the miners.

  13. Bouncing off the Wall In the overhead view shown below, a 290 g ball with a speed v of 4.6 m/s strikes a wall at an angle of 30° and then rebounds with the same speed and angle. It is in contact with the wall for 11 ms. Overhead View • What is the impulse on the ball from the wall? • What is the average force (magnitude & direction) on the wall from the ball?

  14. Examples of Collisions and Explosions Oil and Water example What does conserved mean?

  15. The Law of Conservation of Momentum In terms of the initial and final total momenta:   Pf = Pi In terms of components: Slide 9-18

  16. Perfectly Inelastic Collision Example

  17. Defining your system - system schema Isolated system is when the sum of all external forces is zero.

  18. Slide 9-19

  19. Example A curling stone, with a mass of 20.0 kg, slides across the ice at 1.50 m/s. It collides head on with a stationary 0.160-kg hockey puck. After the collision, the puck’s speed is 2.50 m/s. What is the stone’s final velocity? Slide 9-20

  20. Inelastic Collisions For now, we’ll consider perfectly inelastic collisions: A perfectly elastic collision results whenever the two objects move off at a common final velocity. Slide 9-21

  21. Example Jack stands at rest on a skateboard. The mass of Jack and the skateboard together is 75 kg. Ryan throws a 3.0 kg ball horizontally to the right at 4.0 m/s to Jack, who catches it. What is the final speed of Jack and the skateboard? Slide 9-22

  22. Example A 10 g bullet is fired into a 1.0 kg wood block, where it lodges. Subsequently, the block slides 4.0 m across a floor (µk = 0.20 for wood on wood). What was the bullet’s speed? Slide 9-23

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