Energy Work Power

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

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

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

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

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

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

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

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

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

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

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

12. Conversion of Energy • Since CoE states that energy cannot be created or destroyed, • when one kind of type is lost, it must be converted to another type of energy • 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 • An object falling from a tall height, ignore air resistance • Gravitation Potential Energy (GPE) is lost • Kinetic Energy (KE) is gained • loss in GPE = gain in KE

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

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

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

17. Skate Park Demo

18. KE and GPE • KE = ½ mv2 • m = mass of object • v = velocity of object • GPE = mgh • m = mass of object • g = gravitational field strength, 10 ms-2 • h= height above the ground

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

20. Example 5a • 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. • Loss in GPE = mgh • = (5)(10)(10) • = 500 J (3 sf)

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

22. Example 5c • What is the velocity of the object after it has fallen for 10 m? • KE = 500J • ½ mv2 = 500 • (0.5)(5)v2 = 500 • v2 = 200 • v = 14.1 ms-1 (3 sf)

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

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

25. Example 7a • 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? • Ans: zero. force is not in same direction as the distance moved

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

27. Power • A person/machine is said to be powerful when he can do a lot of work in very little time • 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? • If Zhiyu studied for 2 hours and Andre studied for 1 hour, who did more work?

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

29. Example 8 • 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. • Work Done = Force x Distance = (5)(10) = 50 J • Power = Work Done / Time • = 50/5 = 10.0 W (3 sf)

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

31. Recap • 5 types of Energy • Principle of Conservation of Energy • Conversion of Energy • GPE = mgh • KE = ½ mv2 • Work Done = Force x Distance • 2 Eqns for Power

32. Quiz!!