2. Work, Power & Energy How do they relate? (Stone, Ebener, Watkins)

4. Work Work – In science, work is defined as force times distance. Joule – The unit of work.

5. Work Work is the product of the force on an object and the distance through which the object is moved. Work = Force  Distance W = F x d So… F = ? d = ? The unit for work is the Newton-meter (N·m) which is also called a Joule (J).

6. x dFW 

7. Work is measured in joules (J). 1 joule is equal to a force of 1 N exerted over a distance of 1 m.

8. How can I tell if work was done? A) Was the object moved by a force AND a distance? B) Is there work done by another force C) Are you forcing something top move against the influence of an opposing force? (gravity or friction) D) Are you changing speed? E) Are you transferring energy to another object?

9. Work or No Work

10. A BOULDER FALLS OFF A CLIFF. IT DROPS 2 M. IT HAS A MASS OF 2 KG. …………………….

11. M= 10kg Mg = 100N In this case, the weight does (positive) work d = 2 m Work = Mgd = (100N)(2m) Work = 200 Nm =200J M= 10kg

12. Practice Makes Perfect Questions: a)How much work is done when a weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters above the ground? b)How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 Newtons for 15 seconds?

13. Power Power – The rate at which work is done is called power. Watt – The unit of power. It is equal to one joule of work per second.

14. Power Problems 1) It takes 2 s for a car to drive down the street. If it takes 1000 J of work for the engine to move the car, how much power is used? 2) A women pushes a box across the floor with a force of 600 N in 3 s. The displacement is 30 m. How much power does she use?

15. Mechanical Energy When work is done on an object, the object generally has acquired the ability to do work. This "ability to do work" is called energy and it has the same units as work….Joules. Two Types of Mechanical Energy –Potential Energy and Kinetic Energy

16. Potential Energy The energy that is stored is called potential energy. An object’s ability to do work by virtue of its position. I’m learning through osmosis!

17. Potential Energy Potential energy comes from the position of an object relative to the Earth. It is stored energy. Potential energy is not “used” until the object is moved to a lower position.

18. Examples to Practice: 1. A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work? (a) 10 lb weight (b) 20 lb weight (c) same work for each lifting (d) not enough information is given to work the problem (c) same work for each lifting

19. Examples to Practice: 2. A 50 kg kid rolls down a snow covered hill that is 25 m high. What’s the kid’s speed at the bottom of the hill? (a) 22 m/s (b) 22 m (c) 1250 m/s (d) not enough information is given to work the problem

20. Examples to Practice: 3.A 4 kg book is sitting on a 2.4 m high shelf. A) How much PE does sit have? B) How fast would it be traveling the moment before it landed if it fell off the shelf?

21. 3 Types of Potential Energy (PE) Elastic PE Chemical PE Gravitational PE What are they? Examples of each? How are they converted? To what?

22. Gravitational Potential Energy PE = Weight  height PE = m g h Question: –How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

23. Roller Coaster energy Rollar Coasters

24. Kinetic Energy The energy of motion. An object in motion stores energy in this motion. It is the energy of motion. It increases with speed. It also can increase with mass.

25. Kinetic Energy KE is dependent upon velocity: Proportional to the square of your velocity : if velocity doubles what happens to KE? (quadruples) KE is also dependent on mass If mass doubles what happens to KE? (doubles too)

26. Energy Conservation Energy is the ability to do work. It is the ability to do work so it is measured in joules as well.

27. Law of Conservation of energy Energy can never be created or destroyed, just transformed from one form into another. The sum of the potential and kinetic energy will always be the same, although some of the energy may be converted to frictional heat, but it is not destroyed.

28. Law of Conservation of Energy Energy can change forms but cannot be created nor destroyed. In a closed, isolated system, the total ME (KE + PE) remains constant. Examples??

30. PE = 1000J KE = 0J PE = 800J KE = 600J KE = 1000J KE = 200J PE = 400J PE = 0J

31. PE = 500J KE = 0J PE = 100J KE = 400J PE = 0J KE = 500J PE = 100J KE = 400J Energy lost due to friction is actually not a loss; it is just a conversion.

32. PE (initial) = KE (final)

33. Conservation of Energy The total energy of a system can be calculated by using the Potential energy equation for the object at it’s highest position. m = 10 kg 30 m EP = mgh m = g = h = 10 kg 9.8 m/s 2 30 m = 10kg (9.8 m/s 2 ) 30m EP= 2940J What about if the ball fell one step? What would happen to the energy? This is the total energy that the ball will ever have.

34. m = 10 kg 30 m EP = mgh Can you find the potential energy of the ball on this step? m = g = h = 10 kg 9.8 m/s 2 20 m EP= 10kg (9.8 m/s 2 ) 20m EP= 1960J What was the total energy of this system? 2940 J So what happened to the rest of the energy? It was expressed as kinetic energy as the ball fell. How much kinetic energy? Conservation of energy states the total energy of the system should equal all the parts of the energy. ETotal= EP + EK 2940 J = 1960 J + EK EK = 980 J

35. Work/Energy Relationship If you want to move something, you have to do work. The work done is equal to the change in kinetic energy. Work =  KE Work is a transfer of Energy!

36. Work & Energy

37. Example Problem 1. A 100 kg mass is dropped from rest from a height of 1 meter. 2. How much potential energy does it have when it is released? 3. How much kinetic energy does it have just before it hits the ground? 4. What is its speed just before impact? 5. How much work could it do if it were to strike a nail before hitting the ground?

38. 100 kg 1 meter nail

39. Calculating KE & Stopping Times http://www.wikihow.com/Calculate-Kinetic- Energyhttp://www.wikihow.com/Calculate-Kinetic- Energy https://youtu.be/4yWA2Rk8wJM