1. Energy Work Power Textbook Chp 6 pg 103-122

2. What is Energy? O Energy is the ability to do work O Unit: Joules (J)

3. 5 forms of Energy O 3 types of Potential Energy O Elastic Potential Energy O Chemical Potential Energy O Gravitational Potential Energy O Kinetic Energy O Thermal Energy O [All have unit of Joules!]

4. Potential Energy O Potential Energy refers to energy which is stored to do work later O the energy is not doing anything now, but has the potential to do something later O There are 3 types of potential energy in your syllabus: elastic, potential and chemical

5. Elastic Potential Energy O Elastic Potential Energy is the energy stored when an elastic object is stretched or squeezed. O E.g. a stretched rubber band

6. Chemical Potential Energy O Chemical potential energy is the stored energy which can only be released by chemical reactions O E.g. battery, food

7. Gravitational Potential Energy (GPE) O GPE is the energy stored due to the height of the object

8. Kinetic Energy (KE) O KE refers to the energy of moving objects

9. Thermal Energy O Thermal energy is energy due to heat. O Hot objects have high thermal energy O Thermal energy is also created when a moving object is opposed by friction

10. 5 forms of Energy O 3 types of Potential Energy O Elastic Potential Energy O Chemical Potential Energy O Gravitational Potential Energy O Kinetic Energy O Thermal Energy

11. Principle of Conservation of Energy (CoE) O Energy cannot be created or destroyed, but can be converted from one form to another O The total energy of an isolated system is constant

12. Conversion of Energy O Since CoE states that energy cannot be created or destroyed, O when one kind of type is lost, it must be converted to another type of energy O E.g. if Energy A is being converted to Energy B, we say the Loss in Energy A = Gain in Energy B

13. Example 1 O An object falling from a tall height, ignore air resistance O Gravitation Potential Energy (GPE) is lost O Kinetic Energy (KE) is gained O loss in GPE = gain in KE

14. Example 2 O An object sliding across a flat rough surface O Object becomes slower – KE is lost O Where is the energy converted to? O Motion against Friction always results in gain in Thermal Energy O Loss in KE = Gain in Thermal Energy

15. Example 3 O Stretched rubber band is released O Stretched rubber band has elastic potential energy O After it is released, elastic potential energy is lost O It starts to move = kinetic energy is gained O Loss in Elastic PE = Gain in KE

16. Example 4 O An electric toy car running on batteries O Toy Car starts to move when switched on O gain in KE O Batteries lose energy over time O loss in Chemical PE O Loss in Chemical PE = Gain in KE

17. Skate Park Demo

18. KE and GPE O KE = ½ mv 2 O m = mass of object O v = velocity of object O GPE = mgh O m = mass of object O g = gravitational field strength, 10 ms -2 O h= height above the ground

19. Common Mistake!! O For energy equations, the right side of the equation MUST be in SI Units O E.g. for GPE = mgh O Units of m must be in kg (not g) O Units of h must be in m (not cm)

20. Example 5a O An object of mass 5 kg is dropped from rest. Determine its loss in gravitational potential energy after it has fallen 10m. Ignore air resistance. O Loss in GPE = mgh O = (5)(10)(10) O = 500 J (3 sf)

21. Example 5b O What is the KE gained by the object after it has fallen for 10 m? O Loss in GPE = Gain in KE O Loss in GPE = 500 J O Gain in KE = 500 J

22. Example 5c O What is the velocity of the object after it has fallen for 10 m? O KE = 500 J O ½ mv 2 = 500 O (0.5)(5)v 2 = 500 O v 2 = 200 O v = 14.1 ms -1 (3 sf)

23. Work Done O In Physics, work done is to describe when a force has moved an object O Equation: O Work Done = Force x Distance O Distance must be same direction as Force O SI Units for Work Done is Joules (J)

24. Example 6 O A force of 5N pushes an object a distance of 10 m. Determine the work done on the object by the force. O Work Done = Force x Distance O = (5)(10) O = 50.0 J (3 sf) F 10 m

25. Example 7a O Lionel exerts 20 N of force holding a stack of books on his hands. He walks forward for 5 m. What is his work done? O Ans: zero. force is not in same direction as the distance moved

26. Example 7b O Still holding the books, Lionel walks up a flight of stairs to a height of 4 m. What is his work done? O Is distance same direction as force? Yes O Work Done = Force x Distance O = (20)(4) = 80.0 J (3 sf)

27. Power O A person/machine is said to be powerful when he can do a lot of work in very little time O If Zhiyu can study 3 chapters of Physics in 1 hour but Andre can study 4 chapters of Physics in 1 hour, who is the more powerful Physics mugger? O If Zhiyu studied for 2 hours and Andre studied for 1 hour, who did more work?

28. Power O There are two possible equations for Power: O Power = Work Done / time O Power = Energy Spent / time O Units of Power: Watts (W)

29. Example 8 O A force of 5N pushes an object a distance of 10 m for a time of 5 s. Determine the power created by the force. O Work Done = Force x Distance = (5)(10) = 50 J O Power = Work Done / Time O = 50/5 = 10.0 W (3 sf)

30. Example 9 O An electric heater used 3600 J of energy in 30 s. What is the power of the heater? O Power = Energy spent / time O = 3600/30 O = 120 W

31. Recap O 5 types of Energy O Principle of Conservation of Energy O Conversion of Energy O GPE = mgh O KE = ½ mv 2 O Work Done = Force x Distance O 2 Eqns for Power

32. Quiz!!