1. Chapter 10 Work, Energy and Simple Machines
Physics
Chapter 10
Work, Energy and Simple Machines

2. Chapter 10 Work, Energy and Simple Machines
10.1 Energy and Work
10.2 Machines

3. 10.1 Energy and Work
Energy
The ability for an object to produce a change in itself or in its surroundings
Units-Joules

4. 10.1 Energy and Work Energy Many forms of energy: Thermal Chemical
Electrical
Mechanical

5. 10.1 Energy and Work Energy Two types of Mechanical Energy
Kinetic energy (KE)
Energy of motion
Potential energy (PE)
Stored energy
(more on this in Chapter 11)

6. 10.1 Energy and Work
Work
The process of changing the energy of a system
The transfer of energy by mechanical means
Units-Joules (energy)

7. W = F x d 10.1 Energy and Work Work To calculate work: Where:
W = work (J = Nm)
F = force (N)
d = distance (m)
also W = mad
also W = mgd

8. 10.1 Energy and Work Work Example:
If a weight lifter applies 150 N of force to a barbell to raise it 0.8m, how much work does she do?
Answer:
120 J

9. 10.1 Energy and Work W = E Work-Energy Theorem
Doing work on an object will increase or decrease its energy
Work causes a change in energy that is equal to the work done
This is the Work-Energy Theorem
W = E

10. 10.1 Energy and Work
Work
Since work is the transfer of energy by mechanical means, there is a direct relationship between work and energy as long as force applied to do the work is in the same direction as the motion of the object.

11. 10.1 Energy and Work
Work
When the force that does the work is applied at an angle, only the portion of the work that is in the direction of motion is used to calculate the work done.

12. 10.1 Energy and Work
Work
When the person pushes the lawn mower only the horizontal component (Fh) of the force applied (Fa) is used to calculate the work done.
So…. W = F cos d
Unless the lawn mower is lifted!!! Fa Fv Fh

13. 10.1 Energy and Work Work Example:
A police dog applies 215N of force on his leash at a 35° angle to the ground. If he pulls his handler 28m, how much work does he do?
Answer
4931 J

14. 10.1 Energy and Work
Work
Under which set of circumstances is work done?
Picking up an apple
Carrying an apple
Putting down an apple
Dropping an apple
Eating an apple
Who does the work?

15. 10.1 Energy and Work What kind of work does friction do?
Is there such a thing as negative work?

16. 10.1 Energy and Work Power The rate of doing work
The rate at which energy is transferred
Watt is the unit for power (W = J/s)

17. 10.1 Energy and Work P = W/t Power Can be calculated by the equation:
P = Fd/t
P = mad/t =mgd/t

18. 10.1 Energy and Work Example:
If a motor applies 1500 N of force to a rollercoaster car to lift it up a 22m hill in 0.60 minutes, what is the power it produces?
Answer:
917 W

19. 10.1 Energy and Work
Homework:
P.242; 19-28

20. 10.2 Machines Machine A device that makes work “easier”
It aids in the transfer of energy from one place to another
It is not a source of energy
It will not increase the work done/energy transferred
It only changes the direction and/or magnitude of the force applied to the machine

21. 10.2 Machines
Machine
The work that is done on the machine (work you do) is called the input work (Win)
Input work is equal to effort force (Fe) times effort distance (de)
Win = Fe x de
Effort force is force put into the machine
Effort distance is distance you move by using the machine

22. 10.2 Machines
Machine
The work that is by the machine (work machine does) is called the output work (Wout)
Output work is equal to resistance force (Fr) times resistance distance (dr)
Wout = Fr x dr
Resistance force is force put out by the machine
Resistance distance is distance the machine moves the object

23. 10.2 Machines Simple Machine A machine is in its simplest form
There are two general types of simple machines:
Inclined Plane
Lever

24. 10.2 Machines Inclined Plane
A simple machine made up of a sloping plank

25. 10.2 Machines Inclined Plane Win = Fe x de dr de Fe
Fr = wt = mg (object)
Wout = Fr x dr

26. 10.2 Machines Inclined Plane
A simple machine that makes work easier by increasing the force out of the machine by increasing the distance the force must be applied into the machine
So Fe < Fr and de > dr
It can also change the direction of the force

27. 10.2 Machines Inclined Plane Other types of inclined planes Wedge
Screw

28. 10.2 Machines
Lever
A simple machine made up of “lever arms” that rotate around a fulcrum

29. 10.2 Machines
Lever
Win = Fe x de
Wout = Fr x dr

30. 10.2 Machines
Lever
A simple machine made up of “lever arms” that rotate around a fulcrum
Can either increase or decrease the effort force by decreasing or increasing the effort distance
Also it can change the direction of the forces

31. 10.2 Machines
Lever
Other types of levers
Pulley
Wheel and Axle

32. 10.2 Machines How machines work A machine just transfers energy
Ideal machine—no energy is lost (?), all the energy put into the machine comes out of the machine
So since the energy is transferred 100% by the machine Win = Wout

33. 10.2 Machines Win = Wout Fe x de = Fr x dr
How machines work (ideal machine)
Win = Wout
Fe x de = Fr x dr
So if the machine increases the effort force (Fe < Fr) it must decrease the effort distance (de > dr) to keep work input equal to work output

34. 10.2 Machines Fe x de = Fr x dr can be rewritten as: Fr/Fe = de/dr
How machines work (ideal machine)
Fe x de = Fr x dr can be rewritten as:
Fr/Fe = de/dr
Ideal mechanical advantage: shows how a machine changes the motion (distance moved) of the forces applied to it.
--it’s the property of a machine (how its designed)
—IMA = de/dr

35. 10.2 Machines Fr/Fe ≠ de/dr How machines work (real machine)
Does Fe x de = Fr x dr for an real machine? Huh?
Fr/Fe ≠ de/dr
Mechanical advantage—how much a machine changes the force applied to it
--you must use machine to determine MA!
Mechanical advantage—MA = Fr/Fe

36. 10.2 Machines
But in real machines some energy is always lost by the machine
So Win > Wout always
Because of this we can judge how “good” a machine is by determining its efficiency

37. 10.2 Machines Efficiency = Wout/Win (x 100%)
Efficiency—the percentage of energy that is transferred out of a machine
Efficiency = Wout/Win (x 100%) Or
Efficiency = MA/IMA (x 100%)

38. 10.2 Machines Lets try some practice problems:
How much force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high? (assume it is frictionless—ideal machine)

39. 10.2 Machines
How much force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high? (assume it is frictionless—ideal machine)
Answer: 20 N
What is the machine’s ideal mechanical advantage?

40. 10.2 Machines
How much force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high? (assume it is frictionless—ideal machine)
Answer: 20 N
What is the machine’s ideal mechanical advantage?
Answer: 2.25
This is also its mechanical advantage (why?)

41. 10.2 Machines Lets try some practice problems:
30 N of force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high.
What is the machine’s ideal mechanical advantage?

42. 10.2 Machines
30 N of force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high.
What is the machine’s ideal mechanical advantage?
Answer: 2.25
What is its mechanical advantage?

43. 10.2 Machines
30 N of force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high.
What is the machine’s ideal mechanical advantage?
Answer: 2.25
What is its mechanical advantage?
Answer: 1.5
What is its efficiency?

44. 10.2 Machines
30 N of force is needed to push a 45 N box up an inclined plane that is 18m long (diagonal) and 8 m high.
What is the machine’s ideal mechanical advantage?
Answer: 2.25
What is its mechanical advantage?
Answer: 1.5
What is its efficiency?
Answer: 66.7%

45. 10.2 Machines
So remember a machine makes work seem easier not by increasing the amount of work that can be done (or energy) but by increasing the magnitude and/or direction of the force put into the machine (how?)

46. 10.2 Machines Lets try some practice problems: P. 210; 13, 14, 15
Homework
P. 242; 29-31, 48-52