1. Chapter 12
Work and Energy

2. Section 1: Work, Power, and Machines
What is Work?
Work – transfer of energy that occurs when a force makes an object move.
For work to occur, an object must move.
The motion of the object must be in the same direction as the applied force on the object.
Work is done on an object only when a force is being applied to the object and the object moves.

3. Section 1: Work, Power, and Machines
Calculating work: work equals force (in newtons) times distance
SI unit is the Joule = J
1 J = 1 N x m

4. Section 1: Work, Power, and Machines

5. Section 1: Work, Power, and Machines

6. Section 1: Work, Power, and Machines
Power - amount of work done in a certain amount of time
rate at which work is done
power equals work divided by time.
Power is measured in watts (W).
1 W = 1 J/s

7. Section 1: Work, Power, and Machines

8. Section 1: Work, Power, and Machines

9. Section 1: Work, Power, and Machines
Machine – Device that makes doing work easier by:
Increasing the applied force
and/or
Changing the direction of applied force to make work easier.
Same amount of work can be done:
by applying a small force over a long distance.
as can be done by
applying a large force over a short distance.
Otherwise: Increasing distance reduces the amount of force needed to do the work.

11. Section 1: Work, Power, and Machines
Machines help move things that resist being moved.
Two types of Forces involved by machines:
Effort Force/Input Force – force applied to the machine.
Resistance Force/Output Force – force applied by the machine to overcome resistance
KEY: Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine.
Input Work cannot = Output Work
Some energy transferred is changed to heat due to friction.
An ideal machine with no friction would have the same input work and output work.

12. Section 1: Work, Power, and Machines
Ideal Mechanical advantage (IMA) is the number of times a machine multiplies the effort force.
IMA > 1 multiplies force
IMA < 1 Does not multiply force, but increases distance or speed

13. Section 1: Work, Power, and Machines

14. Section 1: Work, Power, and Machines

15. Section 2: Simple Machines
A machine that does work with only one movement is a Simple Machine.
6 Simple machines
Two families of simple machines w/ 3 simple machines each:
Lever Family
Incline plane family

17. Section 2: Simple Machines
Lever family
Lever – A bar that is free to move about a fixed point.
Parts of a lever
Fulcrum – The fixed point of a lever
Effort Arm – The part of the lever that the effort force is applied to.
measured from the fulcrum to the point at which the force is applied.
Resistance Arm – The part of the lever that applies the resistance force.
measured from the fulcrum to the center of the resistance force.

18. Section 2: Simple Machines
IMA = Effort arm / resistance arm

19. Section 2: Simple Machines
Three classes of levers
based on positions of effort force, resistance force, and fulcrum
First-class lever – fulcrum is located between the effort and resistance forces
multiplies and changes direction of force
IMA = 1 or be < 1 or be > 1
Example: Scissors and seesaw

21. Section 2: Simple Machines
Second-class lever – resistance force is located between the effort force and fulcrum
always multiplies force
IMA always > 1
Example: wheelbarrow

23. Section 2: Simple Machines
Third-class lever – effort force is between the resistance force and fulcrum
doesn’t multiply force
does increase distance over which force is applied
IMA always < 1
Examples: Baseball bat, Broom, forearm

25. Section 2: Simple Machines
Grooved wheel with a rope, simple chain, or cable running along the groove is a pulley
which is a modified first-class lever.

26. Section 2: Simple Machines
Three types of Pulleys
A fixed pulley is attached to something that doesn’t move
force is not multiplied but direction is changed
IMA = 1.
A movable pulley has one end of the rope fixed and the wheel free to move
multiplies force
IMA = 2.

27. Section 2: Simple Machines
Block and tackle – system of pulleys consisting of fixed and movable pulleys
IMA = number of ropes supporting resistance weight

29. Section 2: Simple Machines
Wheel and axle – machine with two wheels of different sizes rotating together
modified lever form
MA = radius of wheel / radius of axle

30. Section 2: Simple Machines
Inclined Plane—sloping surface that reduces the amount of force required to do work
IMA = length of slope / height of slope or
IMA = effort distance / resistance distance
Less force is required if a ramp is longer and less steep.

32. Section 2: Simple Machines
Inclined plane with one or two sloping sides is a wedge.
Example: Knife
Thinner wedges < IMA than thick wedge w/ = Length
Screw – inclined plane wrapped in a spiral around a cylindrical post.
IMA increases the closer the threads are together.

34. Section 2: Simple Machines
Compound machine – uses a combination of two or more simple machines

35. Section 3: What is Energy?
Energy is the ability to cause change.
Ability to do work.
Joule – The SI unit used to measure energy
Work and Energy have same unit.
Work is the transfer of energy.
Energy can be stored.
Energy only seen when it is transferred

36. Section 3: What is Energy?
Potential energy – Energy stored in a motionless object, giving it the potential to cause change
Elastic potential energy (EPE) – Energy stored by things that stretch or compress.
Gravitational potential energy (GPE) – Energy stored by things that are above Earth .

37. Section 3: What is Energy?

38. Section 3: What is Energy?
The amount of GPE an object has depends on its mass , the acceleration due to gravity, and its height above ground.
GPE = mass in kilograms x 9.8 m/s 2 x height in meters

39. Section 3: What is Energy?

41. Section 3: What is Energy?
Kinetic energy – Energy in the form of motion.
The amount of kinetic energy an object has depends on its mass and its velocity.
Kinetic energy = 1/2 mass x velocity 2

42. Section 3: What is Energy?

45. Section 3: What is Energy?
Mechanical energy – the total amount of potential and kinetic energy in a system
Non mechanical energy – Energy that does not affect motion.
Atoms have kinetic energy.
Chemicals can store PE in their bonds
All living things get energy from sun.

46. Section 3: What is Energy?
Sun uses Nuclear reaction to produce energy.
Electricity a form of energy
Light carries energy in the form of a wave

47. Section 4: Law of Conservation of Energy.
Roller coasters and Pendulums demonstrates Law of Conservation of Energy.
Law of Conservation of Energy – Energy may change from one form to another, but the total amount of energy never changes.
1st law of thermodynamics – Energy in universe is constant.

48. Section 4: Law of Conservation of Energy.
Roller coaster is pulled to top of hill by conveyer belt.
PE is at its max at top of hill.
Once in motion PE transfers to KE.
Once at top of next hill KE has been transferred back to PE.

51. Section 4: Law of Conservation of Energy.
Due to air resistance and friction energy is lost in other forms like sound and heat.
Roller coaster will never reach a height of the first hill.

52. Section 4: Law of Conservation of Energy.
Pendulum:
As a swing moves back and forth, its energy continually converts from kinetic to potential and back.

54. Section 4: Law of Conservation of Energy.
Scientist study energy systems.
Closed System – energy lost to surrounding is near zero.
Open system – energy is free to transfer to surrounding.

55. Section 4: Law of Conservation of Energy.
No machine is 100 % efficient
Efficiency is a measure of useful work done by a machine.
The useful work output of a machine never = or exceeds the work input.

56. Section 4: Law of Conservation of Energy.

57. Section 4: Law of Conservation of Energy.

59. Chapter 13
Thermal Energy

60. Section 1 Temperature and Heat
A. Temperature—related to the average kinetic energy of an object’s atoms or molecules
B. Thermal energy—the sum of the kinetic and potential energy of all the atoms in an object
1. Thermal energy increases as temperature increases.
2. At constant temperature, thermal energy increases if mass increases.

62. Thermal energy that flows from something at a higher temperature to something at a lower temperature is called heat.

63. Energy Transfer
A. Conduction—transfer of thermal energy through matter by direct contact of particles
1. Kinetic energy is transferred as particles collide.
2. Solids, particularly metals, are good heat conductors.

65. Energy Transfer
B. The transfer of energy by the motion of heated particles in a fluid is called convection.
1. Convection currents transfer heat from warmer to cooler parts of a fluid.

67. Energy Transfer
C. Radiation – energy transfer by electromagnetic waves
1. Some radiation is absorbed and some is reflected when it strikes a material.
2. Heat transfer by radiation is faster in a gas than in a liquid or solid.

70. D. Insulator – material that does not let heat flow through it easily