1. Work, energy, and power

2. If the net force is in the direction of motion work is positive If the net force is in the direction opposite that of motion work is negative If the net force is perpendicular to the direction of motion work is 0

3. Work, energy, and power Work done by a variable force Plot the force vs displacement graph The area between the line and the horizontal axis will be the work done

4. Figure 7-10 Work Needed to Stretch a Spring a Distance x

5. Work, energy, and power System – a collection of objects – part of the universe that we are studying. Work – a measure of energy transfer between systems. Energy is the ability to do work.

6. Work, energy, and power

8. Example of ball thrown in the air.

9. Work, energy, and power Potential energy is stored energy by virtue of an objects position or its configuration. At least two objects must be interacting. Symbol is U Unit; joule (J)

10. Work, energy, and power When working with potential energy we only consider changes in energy. We can define 0 potential energy at any position that is convenient to us.

11. Work, energy, and power

14. Falling ball example.

15. Work, energy, and power

16. State of a system Initial state – system at the beginning of a process. Final state – system at the end of a process. The energy change of a system only depends on its initial and final state.

17. Work, energy, and power

18. Conservative and Nonconservative forces. Work done with conservative forces does not result in loss of mechanical energy. Work done with nonconservative forces results in loss of mechanical energy.

19. Table 8-1 Conservative and Nonconservative Forces ForceSection Conservative forces Gravity5-6 Spring force6-2 Nonconservative forces Friction6-1 Tension in a rope, cable, etc.6-2 Forces exerted by a motor7-4 Forces exerted by muscles5-3

20. Figure 8-3 Work Done by Gravity on a Closed Path

21. Figure 8-4 Work Done by Friction on a Closed Path

22. Work, energy, and power

23. Figure 8-6 The Work Done by a Conservative Force Is Zero on Any Closed Path