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AP Physics I.D

AP Physics I.D. Impulse and Momentum. The Hummer and the Smart Car. Another look at the Second Law. “The alteration of motion is proportional to the force impressed” – Isaac Newton (as related in a private conversation to W. Evans). Momentum. Product of mass and velocity

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AP Physics I.D

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  1. AP Physics I.D Impulse and Momentum

  2. The Hummer and the Smart Car

  3. Another look at the Second Law

  4. “The alteration of motion is proportional to the force impressed” – Isaac Newton (as related in a private conversation to W. Evans)

  5. Momentum • Product of mass and velocity • It’s a vector Victor p = mv

  6. Impulse (a vector and has same direction as average force)

  7. So you can see, a large force acting for a short period of time can create the same change in momentum as a small force acting for a large period of time.

  8. Examples of impulse • Two AP science teachers vs. a doubles team headed for state • Daniel Sepulveda • Bunting a baseball • Dick Fosbury • Air bags • Bending your knees • Boxing • An egg and sheet

  9. Ex. A 80.0 kg stuntman jumps out of a window that 45 m above the ground. a) How fast is he falling when he reaches ground level? b) He lands on a large, air filled target, coming to rest in 1.5 s. What average force does he experience while coming to rest? c) What if he had landed on the ground instead, coming to rest in 10.0 ms? (ouch!)

  10. Ex. An object with mass 3m moves along the +x-axis with a speed v. A second object with mass 2m and speed 2v moves along the –y-axis with a speed of 2v. What is the magnitude of the net momentum of the two objects?

  11. Graphing force vs. time

  12. Conservation of Linear Momentum

  13. Two types of forces on the system • Internal – forces that objects within the system exert on each other • External – forces that objects outside the system exert on each other

  14. Linear momentum is conserved if the sum of the external forces is zero and the system is isolated. The momentum before the event is equal to the momentum after the event.

  15. Ex. An astronaut is floating in space near her shuttle when she realizes that the cord that attaches her to the ship has become disconnected. Her total mass (body, suit and equipment) is 91 kg. She reaches into her pocket, finds a 1.0 kg metal tool, and throws it directly away from the ship with a speed of 9.0 m/s. If the ship is 10.0 m away, how long will it take her to reach it?

  16. Rifle Recoil: Find the recoil velocity of a 5.0 kg rifle that shoots a 0.050 kg bullet at a speed of 120 m/s.

  17. Collisions (and explosions) in One Dimension

  18. Elastic collision – momentum and kinetic energy are conserved (objects bounce off one another – in real life this only happens at the atomic level)

  19. Inelastic collision – momentum is conserved, but kinetic energy is not

  20. Completely inelastic collision – all kinetic energy converted to other forms of energy – the objects stick together (thud)

  21. Note: momentum is conserved if no external forces act on the system, but kinetic energy is conserved only in elastic collisions.

  22. Ex. Two balls roll toward each other. The red ball has mass of 0.50 kg and a speed of 4.0 m/s just before impact. The green ball has a mass of 0.20 kg and a speed of 2.0 m/s. After the head-on collision, the red ball continues in its initial direction with a speed of 2.0 m/s. Find the velocity of the green ball after the collision. Is the collision elastic or inelastic?

  23. Ex. Two blobs of clay are thrown towards each other and collide in mid-air. The red clay has a mass of 0.50 kg and a speed of 4.0 m/s just before impact. The green ball has a mass of 0.30 kg and a speed of 2.0 m/s. If the collision is completely inelastic, determine the velocity of the composite object immediately after the collision.

  24. Ex. A bullet with a mass of 0.0100 kg strikes, and is embedded in a block (mass of 2.50 kg) that is suspended by a thin cord. The block rises a vertical height of 0.650 m because of the collision. What is the initial velocity of the bullet?

  25. Collision in Two Dimensions If no external forces act on a system, horizontal and vertical components of momentum are conserved.

  26. Ex. A 500 kg car travels at 20.0 m/s due north. It collides with a 500 kg traveling due west at 30.0 m/s. The cars lock bumpers and stick together. What is the velocity of the combined cars immediately after impact?

  27. Ex. An object of mass m moves with velocity v toward stationary object of mass 2m. After impact, the objects move off in directions as shown. a) Determine the magnitudes of the velocities after the collisions in terms of v. b) Is this an elastic or inelastic collision?

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