Chapter 7 Work, Energy and Power (Chap. 6 in the textbook page 139)

1. Chapter 7Work, Energy and Power(Chap. 6 in the textbook page 139) Dr. Haykel Abdelhamid Elabidi April 2014/Ju T 1435

2. Units of Chapter 7 (6) • Work of a constant force • Kinetic energy • Potential energy and conservative forces • Dissipative forces • Power

3. Work done by a constant force

4. Work done by a constant force Example 6.1. page 140: A 600 N force is applied by a man to a dresser that moves 2 m. Find the work done if the force and displacement are parallel; b) at right angles; c) oppositely directed; we may imagine that the dresser is being slowed and brought to rest. Solution 6.1. page 140:

5. Work done by a constant force Example 6.2. page 140: A horse pulls a barge along a canal with a rope in which the tension is 1000 N (Fig). The rope is at an angle of 10⁰ with the towpath and the direction of the barge. How much work is done by the horse in pulling the barge 100 m upstream at a constant velocity? What is the net force on the barge?

6. Kinetic energy

7. Kinetic energy Example 6.3. page 142: Solution 6.3. page 142:

8. Potential Energy In general, potential energy is energy associated with the position or configuration of a mechanical system. In Fig. 8, a ball rises from initial height h0 to a height h. The gravitational force mg is opposite in direction to the displacement s = h – h0, so the work done is negative: W(grav)= - mg (h-h0) The change in potential energy is : ∆ U = U – U0 The magnitude of ∆ U is defined to be equal to the magnitude of W(grav): U – U0 = - W(grav) This result for potential energy change involves a difference of two terms on each side, and suggests that we define the potential energy themselves at h and h0, respectively, by U = mgh and U0 = mgh0 Figure 8

9. Potential energy

10. Potential energy Example 6.5. page 144: A woman skis from rest down a hill 20 m height (Fig. 9a). If friction is negligible, what is her speed at the bottom of the slope?

11. Potential energy

12. Dissipative forces The frictional forces are treated as applied forces: they give negative work. The work of these forces are converted in thermal energy: dissipative forces. Example 6.6. page 145: A woman skies from rest down a hill 20m high. If friction is not negligible and if her speed at the bottom of the slope is only 10 ms-1, how much work is done by frictional forces if her mass is 50kg.

13. Dissipative forces

14. Conservative forces Any force that has the property that its work is the same for all paths between two given points is said to be a conservative force. Gravitational, electrical and spring forces are conservative forces. Friction and many other forces are not conservative.

15. Power The power is defined as: , the SI unit J/s= watt (W)

16. Power Example 6.14. page 152: A 70-kg man runs up a flight of stairs 3m high in 2s. (a) How much work does he do against gravitational forces? (b) what is his average power output? Solution Example 6.14. page 152:

17. Homeworks: Exercises 6.2; 6.21; 6.36; 6.45 Thank you for your attention See you next time Inchallah