2. 1 7.1 Work and energy 7.1 Work and Energy

3. 2 7.1 Work and energy When a force moves something, work is done. In everyday language, work may mean anything that people do. 1Work aMeaning of work

4. 3 7.1 Work and energy What is work then? In physics, work is done whenever A transfers energy to B by forcing B to have a displacement. ameaning of work

5. 4 7.1 Work and energy force in the direction of displacement  displacement W = F // s F s Work is calculated by: ameaning of work Work =

6. 5 7.1 Work and energy 3 Energy transfer 2 Displacement of object 1 Application of force ameaning of work 3 things happen when work is done:

7. 6 7.1 Work and energy  Scalar (has magnitude only, no direction)  Unit: N m or J 1 J of work is done when a force of 1 N moves a distance of 1 m in the direction of the force. Work ameaning of work

8. 7 7.1 Work and energy ameaning of work If there is no motion along the force direction, no work is done on the object even though a force is exerted on it. F E.g.: Carrying a load is tiring, but no work is done on it. Why? There is no displacement (s = 0) along the direction of force. standing still

9. 8 7.1 Work and energy When force makes  with displacement,  only component of the force in the direction of displacement counts bwork (non-parallel cases) E F W = Fs cos   this component // forward motion F F cos  F sin   s W = F // s = (F cos  )s = F (s cos  ) = Fs //

10. 9 7.1 Work and energy W = Fs cos  When  = 0 o, W = Fs When  = 90 o, W = 0 When  = 180 o, W = -Fs

11. 10 7.1 Work and energy the sign of work does not indicate direction. +ve work:  in energy of the object.  ve work:  in energy of the object. 3Energy transferred and work Sign of work: Work done by resistive force e.g. friction Energy is dissipated in the form of heat and sound.

12. 11 7.1 Work and energy  KE of car  e.g. when braking a car, the braking force does  ve work on the car. F S 3Energy transferred and work Sign of work: F

13. 12 7.1 Work and energy Energy Energy exists in many different forms. But we shall only study the different forms of mechanical energy. 2Mechanical energy Elastic potential energy Gravitational potential energy kinetic energy

14. 13 7.1 Work and energy  A moving object has kinetic energy.  The faster it moves, the more KE it possesses. chemical energy kinetic energy aKinetic energy (KE)

15. 14 7.1 Work and energy When you lift a box up, it gains gravitational potential energy ( 重力勢能 ). bGravitational potential energy (PE) put the box higher box possesses more PE

16. 15 7.1 Work and energy Another form of potential energy cElastic potential energy ( 彈力勢能 ) When an elastic object is stretched, compressed or bent object gains elastic potential energy

17. 16 7.1 Work and energy 3Energy transferred and work Work is done whenever energy is transferred from one form to another. energy in one form work done energy in another form

18. 17 7.1 Work and energy work done 10 J potential energy 10 J kinetic energy 10 J energy in one form work done energy in another form Work done = energy transferred 3Energy transferred and work

19. 18 7.1 Work and energy A man lifts a 10-kg suitcase on his shoulder through a vertical distance of 1.5 m. Example 1 Work done in lifting a suitcase 1.5 m 10 kg (a)How much work does he do? What is the energy transfer in this process? (b)Does he do work when he transports the suitcase by carrying it on his shoulder?

20. 19 7.1 Work and energy (a)Force = weight of suitcase = 10  10 = 100 N Work = 100 × 1.5 = 150 J Chemical energy of the man is converted to the potential energy of the suitcase. Example 1 Work done in lifting a suitcase 1.5 m 10 kg 100 N

21. 20 7.1 Work and energy (b)Force on suitcase is at right angle to the motion. Example 1 Work done in lifting a suitcase 5 m Displacement in the direction of force is zero.  work done is zero.

22. 21 7.1 Work and energy How much work does he do? What is the energy transfer in this process? Example 2 E Work done in pulling a suitcase 30  20 N 30  20 N 5 m Component of force in the direction of motion = (F cos  ) s = 20 cos 30  = 20 cos 30   5 = 86.6 J Man's chemical energy  suitcase's KE 10 kg Work

23. 22 7.1 Work and energy Work done by tension of the string = 0

24. 23 7.1 Work and energy AWork = applied force in the direction of displacement  displacement BSince displacement and force are vectors, work done is also a vector. CWork is done when a force moves something. DOur muscles do work when we walk. Which of the following statements about work is INCORRECT? Q1Which of the following...

25. 24 7.1 Work and energy d A block of weight W slides down an inclined plane over a distance d. AWork = W  d  sinθ BWork = W  d  cosθ CWork = W  d E Find work done by gravity on the block. Q2A block of weight W... θ

26. 25 7.1 Work and energy The following screw jack is used to raise heavy objects by turning the handle. 50 kg load handle 30 cm Q3The following screw...

27. 26 7.1 Work and energy What is the work done in one complete revolution if the turning force is 20 N? = _______  2    ________ = ________________ J 200.3 37.7 The work done of the turning force = force × displacement Q3The following screw... 30 cm

28. 27 7.1 Work and energy The End

29. 28 7.1 Work and energy Return