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Plan for Today (AP Physics 1)

Plan for Today (AP Physics 1). Go over tests Lecture/Notes on Work and Energy. Test Results. Average: 86.6 Without the curve: 82 percent 25 As. Students do the following. Curl a bowling ball 10 times Take a bowling ball to the water fountain and back Push a brick across a table. Vote.

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Plan for Today (AP Physics 1)

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  1. Plan for Today (AP Physics 1) • Go over tests • Lecture/Notes on Work and Energy

  2. Test Results • Average: 86.6 • Without the curve: 82 percent • 25 As

  3. Students do the following. . . • Curl a bowling ball 10 times • Take a bowling ball to the water fountain and back • Push a brick across a table

  4. Vote • Which student did the most work? • Only one to do work

  5. Equation • Work = Force times distance times cos angle • Work = F x cos 0 • Units: • N*m • Kgm/s^2 *m = kgm^2/s^2 • Joules – energy • This is a scalar quantity • No direction • Note: Physics definition of work • Very narrowly defined

  6. Why do you need cos in the equation? • Force must be in the DIRECTION of motion

  7. Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?

  8. Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?

  9. Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?

  10. Situations • Carry a brick across the room • Was any work done? • No • Force and displacement in different directions • No change in its energy • Velocity and height don’t change

  11. Situations • Carry a brick across the room • Was any work done? • No • Force and displacement in different directions • No change in its energy • Velocity and height don’t change

  12. Situation • What if I pick a brick straight up? • Force of hand is like the normal force • In same direction as the distance traveled • So yes, work is done • Work is positive

  13. Situation • What if I pick a brick straight up? • Force of hand is like the normal force • In same direction as the distance traveled • So yes, work is done • Work is positive

  14. Situation • Pushing a rolling chair and allowing it to roll to a stop • What happened? • I put energy into the chair • Force times distance applied = work • What happens to the energy? • From my work it converts to kinetic energy • What happens to KE? • Does this energy disappear?

  15. Chair situation continued • What happens to the kinetic energy? • We did work, it had kinetic energy, but then it stopped • Work done • Kinetic energy converted to what? • Heat from friction

  16. Chair situation • The work done by friction opposed motion • So the work is negative

  17. Generalization • Negative Work slows down • Positive work speeds up

  18. Example • A 10 kg mass moves 5 m. The coefficient of friction is .2. • What is the net work if a force of 40 N is applied at 30 degrees above horizontal?

  19. Work KE Theorem

  20. Work KE Theorem

  21. Work KE Theorem

  22. Work KE Theorem

  23. Work KE Theorem

  24. Work KE Theorem

  25. Work KE Theorem

  26. What did this mean? • What is ½*m*v^2? • KE = ½*m*v^2 • Are we sure this is correct? • How should we check? • Units! • KE = (kg)(m/s)^2 =kgm^2/s^2 • What is this? • Joules!

  27. Example • A car with a mass of 1000 kg starts at 10 m/s and accelerates to 20 m/s in 50 m. • How much work is done? • What force is required?

  28. 150000 • 3000

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