2. WORK AND ENERGY

3. WORK
The work done by a constant force acting on an object is equal to the product of the magnitudes of the displacement and the component of the force parallel to that displacement.
W = F • Δx
W = F*d cosq

4. Units of Work .Newton • meter A Newton • meter is called a Joule
1 Joule = 1 Newton • meter
1 J = 1 N • m = 1.36 ft-lb

7. THE SIGN of WORK
Where F is the “resultant force” causing work and d is the displacement.

9. Who’s Doing Work? A B C D

10. Try this problem
A horse pulls a stationary 300 kg cart (which was initially at rest) for 55 m. The horse pulls on the cart at an angle of 35o with a 450 N constant force.
Find the force along the direction of motion.
Find the work done by the horse.
Find the acceleration of the cart.
Find the total time this takes.

11. POWER ! Power is the rate of doing work.
Power is measured in units called Watts

12. Units of Power
The units of power are:

13. ENERGY
One of the most impossible to describe concepts in all of physics
Most tetbooks say it is the capacity for a system to do work onto, or transfer heat with its surroundings
We will mainly talk about work and energy in this unit, but along with heat they all have the same unit: JOULES!
ENERGY
WORK
HEAT

14. The Inclined Plane A Simple Machine

15. Independence of Path Work is the same Force applied mg Length of ramp
Height

16. Forms of Energy

17. Chemical Energy

18. Solar Energy

19. Electrical Energy

20. Wind Energy

21. Sound Energy

22. Nuclear Energy

23. Heat Energy

24. Mechanical Energy

25. Work-Energy Theorem W=F•d W=ma•d vf2 = vi2 + 2ad and solve for ad
The result of work is that there is a change of speed, or an acceleration.
W=F•d
W=ma•d
vf2 = vi2 + 2ad and solve for ad
ad = (vf2 – vi2)/2
Insert into W=mad
W=m(vf2 – vi2)/2

26. Work-Energy Theorem

27. Kinetic Energy
Kinetic Energy is energy of motion.

28. Work-Energy Theorem

29. EXAMPLE
A block of 2 kg is initially at rest. If a force of 10 N pushes the block for a distance of 5 m, what is the final velocity of the block?

30. Gravitational Potential Energy
The force of gravity does work. It accelerates objects.
W = Fg • d
W = mg • d
W = mg • h
(h = height above a reference point)

31. Gravitational Potential Energy
Note: AP Physics uses the symbol U for potential energy

32. Potential Energy and Work
When work is done by gravity or against it, the change in PE is equal to the work done.

33. Work and Gravity

34. Work and Friction

35. Conservation of Energy
Energy is conserved in all systems. This means that in the Universe, all the energy that was at the beginning of time is still in the Universe today.
As we isolate systems, we find that the total energy before is equal to the total energy after.
The energy just changes form.

36. Mechanical Energy

37. Spring Potential Energy
Springs possess potential energy if they are elongated or compressed from their rest positions.
This is called spring (or elastic) potential energy.
The formula is derived from Hooke’s Law:

38. Spring Potential Energy

39. Try this Problem
Find the final speed of the object as it hits the pavement.
35m

40. The Roller Coaster

41. Conservation of Energy- The Roller Coaster

42. The Roller Coaster ProblemF D
Find the Speeds at points A-F

43. Atwood’s Machine Revisited

47. Conservative Force
A conservative force is such that the work done on the object is independent of the path. OR
The force does NO work as the object moves around a closed path from start to finish.

48. Conservative vs. Non-conservative Force
Gravity
Elastic Spring
Electric
Non-conservative
Friction
Air resistance
Tension
Normal force

49. Conservative Force

50. Non-Conservative Force

51. Internal Energy
The total energy contained within a thermodynamic system.
Basically we are talking about heat when we use the term internal energy
Usually in mechanical systems, heat is created because of friction, air resistance, or water resistance
Energy in this form is typically wasted energy

52. Non-Conservative Forces
A force is non-conservative if the work done on an object is dependent upon the path of travel.

53. Review Questions
Write responses to each question on a separate sheet of paper to be handed in.
There are 6 questions.
Some are equations, some require written responses and deep thought, while some are graphical.

54. Question 1
What is the definition of Work in Physics?

55. Question 2
Name two ways in which Work and Energy are related.
Energy
Work

56. Question 3
What equation equates Work with Kinetic Energy?

57. Question 4
Explain how this picture shows that:

58. Question 5
How does the statement, “the Earth gets all its energy from the Sun,” explain how you can be awake here today?

59. Question 6
Draw a graph that shows how PE and KE change while riding this roller coaster Label points W, X, and Y on the graph.

60. In Summary Work is Force times a displacement.
Kinetic Energy is 1/2 mv2.
Gravitational Potential Energy
is mgh.
The work-energy theorem is
W=  K.
Conservation of energy
Spring potential energy is 1/2 kx2