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Lecture 14 Monday, September 29

Lecture 14 Monday, September 29. Using Newton’s Laws. Harper’s Index. Minimum number of U.S. cities that have shortened the yellow light on traffic signals to under the legal limit: 6. Today. Equilibrium F NET = 0 Dynamics F NET ≠ 0

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Lecture 14 Monday, September 29

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  1. Lecture 14Monday, September 29 Using Newton’s Laws

  2. Harper’s Index • Minimum number of U.S. cities that have shortened the yellow light on traffic signals to under the legal limit: • 6

  3. Today • Equilibrium FNET = 0 • Dynamics FNET ≠ 0 • Mass and Weight • Normal forces • Frictional forces • Chapter 5: 1-5

  4. Equilibrium • Hence a=0 and v=constant • If v=0 and is constant, static equilibrium • If v≠0 and is constant, dynamic equilibrium

  5. 300 Find the tensions, T1 and T2 in the two cords if the mass of the picture is 11.1 kg.

  6. Procedure • Make free body diagram • Picture in (static) equilibrium so

  7. Example Macie pulls a 40 kg rolling trunk by a strap angled at 30° from the horizontal. She pulls with a force of 40 N, and there is a 30 N rolling friction force acting on trunk. What is the trunk’s acceleration? Slide 5-17

  8. Mass and Weight • Freely falling object

  9. Elevators • You are standing on a scale in an elevator. • A) The elevator is moving upwards (downwards) at a steady speed. If your mass is 70 kg, what does the scale read? • B) The elevator which had been moving upwards, slows to a stop. What does the scale read during this time? • Normal force

  10. Quiz • An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. Make a free body diagram. • Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-23

  11. Friction • Static friction • Opposes motion

  12. Kinetic Friction • Opposes motion when object is sliding • µk is the coefficient of kinetic friction

  13. Rolling Friction • µr is the coefficient of rolling friction

  14. Problem • 5:24 A 23kg child on slide inclined at 380. • What forces act on child? • Make a free body diagram • Magnitude of normal force? • If the coefficient of static friction is 0.62, will the child start to slide?

  15. Problems due Today • 4: 13, 14, 17, 18, 20, 25, 29, 30, 31, 33, 35, 38, 65

  16. Problems due Wednesday • Reading: 5: 6-8 Drag (skip), applying Newton’s laws • Problems: 5: CQ1, CQ3, MC24, MC26, MC30, 1, 2, 4, 7, 11, 13, 16, 19, 21

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