1) Work is done when a force causes an object to move in the direction of the force. Different types of energy include kinetic, potential, chemical and thermal. 2) The principle of conservation of energy states that energy cannot be created or destroyed, only converted from one form to another. 3) Kinetic energy is defined as E_k=1/2mv^2 and gravitational potential energy as E_p=mgh. These relationships can be used to solve problems involving work, energy and power.

1. Work Energy Power

2. Before we move on, We have… Representing the video clips available Representing the websites available Representing the applets available

4. Lesson Trigger Class, have you done your work? I have done my work, Teacher.

5. Work is a force related quantity. Understanding work done A force MUST be applied on an object in order for work to be done. Applying a force to strike the tennis ball – you are doing work on the ball. Applying a force to lift your body weight up the stairs – you are doing work against gravity. Pulling you down during a dive – the Earth is doing work on you. Applying a force to drag the bag – you are doing work against friction.

7. The SI unit for work is the joule (J). To a scientist, work W is done whenever a force F makes an object move a certain distance D in the direction of the force. The greater the force, and the further it moves, the more work is done . Defining Work 1 joule is defined as the amount of work done when a force of 1 N moves an object 1 m in the direction applied force. W ork = F orce  D istance in the direction of applied force 1 J = 1 N m N m

8. (a) Work done against friction Work is done against friction when a force is applied to move an object in contact with a surface over a certain distance. Need to apply a force = 28 N to overcome friction. Nature of Work Done Need to apply a force = weight of the load to overcome gravitational pull. (b) Work done against gravity Work is done against gravity when a force is applied to lift an object to greater height in a gravitational field. 1.5 m 200 N

10. Mr Tan of mass 95 kg is running up to the 5 th floor of his HDB flat, 12.0 m away from the ground floor. How much work does he do against gravity? Mr Tan is moving up against gravity. He has to apply a force = his body weight to overcome the pull of gravity. WD gravity = Force  distance = weight  distance = 950  12.0 = 11 400 J Sample Calculation 2

12. Think about and write down what you know about any kind of energy. Write down whatever that comes to your mind when you think about the term energy. Lesson Trigger Energy Kinetic energy – energy due to motion, – all moving objects have it. Potential energy Electrical energy Nuclear energy Sound energy Heat energy Mechanical energy internal energy

13. Energy is defined as the ability to do work . You make use of different types of energy to help you do work in your everyday life. Defining Energy The SI unit of energy is joule , symbol J .

15. When an object stores energy as the result of its position in a gravitational force field , the object is said to possess Gravitational Potential energy, PE gravity The ram of a pile driver possesses gravitational potential energy. It is capable of doing work on the wooden pole. When released, it applies a force F to move the wooden pole a distance h into the ground. Work done = F  D = mgh = mg Gravitational Potential Energy Example ram wooden pole h F

16. Gravitational Potential Energy ram wooden pole PE gravity = mgh Hence gravitational potential energy PE gravity is given as: mass in kg gravitational field strength g = 10 N/kg height in m The SI unit of PE gravity is joule, symbol J. F = mg h

17. Sample Calculation 3 In a rescue operation, a 75 kg rescuer is raised to a height 12 m above ground. Calculate his gain in Gravitational PE. PE = mgh Quick Practice (3 min) TB pg 121 Q3 = 75  10  12 = 9 000 J

18. Kinetic Energy A body in motion is capable of doing work. It is said to possess Kinetic Energy, KE. A swinging mallet has kinetic energy, When it strikes the ball with a force F , it causes the ball to move a certain distance D in the direction of the force, producing work. W = F  D

20. Sample Calculation 4 (i) Ali jogs at a uniform speed of 7.5 m/s. Calculate his kinetic energy if his mass is 65 kg . (ii) How will his KE change if he slows down his motion? As KE is depending on speed of motion, if he slows down, his KE will decrease. KE = ½ mv 2 = ½  65  7.5 2 V = 1 828.125 J  1.8  10 3 J Quick Practice (3 min) TB pg 121 Q2

21. Laws of conservation of energy The energy of a body is always converted from one form to another during work done. Work is done against gravity when a car is driven up a slope. Chemical E p (petrol) ➔ Gravitational E p (car gains height) If the car accelerates up the hill, then: Chemical E p (petrol) ➔ Gravitational E p (car gains height) + E k (car speeds up) More petrol will be burnt to release more Chemical E p to accelerate the car. Example

23. Laws of conservation of energy Energy cannot be created nor destroyed . It can be converted from one form into another, during which time, work is always done. In any closed system, the total amount of energy remains constant before and after work done, regardless of any process which takes place.

25. Sample Calculation 5 (c) (i) State the assumption you are making in the calculation in (b). (ii) what is the significant of this assuption? E p = mgh = 200  10  1.8 = 3 600 J 1.8 m E k = E p = 3 600 J We assume that air resistance is negligible, no work is done to overcome air resistance. When air resistance is negligible: E p lost = E k gain . Otherwise , E p lost = E k gain + work done against air resistance. Quick Practice (3 min) TB pg 123 Q8

26. Mechanical energy is energy of motion ( E k ) or of potential for motion ( E p ) on a macroscopic scale (a system). Mechanical Energy A catapult flying through air has E k and E p . Is there a special name for such a body possessing two such energies simultaneously? http://www.youtube.com/watch?v=HNkqy-qsheY

28. Power http://youtube.com/watch?v=xzKrSTx4Imo http://youtube.com/watch?v=X666_Y7C_tg How do we measure power?

29. SI unit - joule per second (J/s) Power is a force related quantity. It measures the rate of work done or energy conversion. Power Another unit, watt (W) is also used. 1 W = 1 J/s Power = work done time Energy converted time = J s

30. Mr Tan of mass 95 kg is running up to the 5 th floor of his HDB flat, 12 m away from the ground floor. If he is able to reach the 5 th floor within 35 s, What is the power developed by him? Mr Tan is moving up against gravity. He has to apply a force of at least equal to his body weight to overcome the pull of gravity. His work done = Force  distance = 950  12 = 11 400 J His power = work done  time = 11 400  35 = 325 W Sample Calculation 6

31. Summary Understand the examples of different forms of energy. By the end of this lesson pupils are able to: State the principle of the conservation of energy. Solve problems using the principle of the conservation of energy. State that kinetic energy is E k = ½ mv 2 and gravitational potential energy E p = mgh . Solve problems using the relationships for kinetic energy and potential energy. Solve problems using the relationship work done = force  distance moved in the direction of the force. Solve problems using the relationship power = work done  time taken.