1. Chapter 5 Work and Machines
Page

2. Review Motion Force Distance, time, speed (velocity), acceleration
Speed = rate of change of distance
Acceleration = rate of change of velocity
Force
Force, mass, acceleration
A force is required to change the velocity of an object.
F = ma
Weight is the gravitational attraction on an object
W = mg (g= 9.8 m/s2 the acceleration of gravity)

3. Review of Energy Energy is the ability to do work.
Different forms of energy
Different types of mechanical energy
KE = ½ mv2, PE (GPE)= mgh
Energy is always conserved
Energy may be conserved but it is often changed into a useless form (heat) by friction and air resistance.
But Einstein said, E = mc2

4. Work Work makes something move
Work is the transfer of energy when a force makes an object move.
Two conditions
A force must make something move
The direction of motion must be in the direction of the force
Carrying books
Work is a transfer of energy
W= Fd (force in Newtons (kg m/s2, distance in m)

5. Practice Problems P 128:1,2,3
You push a refrigerator with a force of 100N. If you move the refrigerator a distance of 5m while you are pushing, how much work is done?
Given:
Asked: ?units
Formula:

6. Given: Asked: Formula: W=Fd Substitute: Answer:
Page 128 Problem 1 A couch is pushed with a force of 75N and moves a distance of 5m across the floor. How much work is done in moving the couch?
Given:
Asked:
Formula: W=Fd
Substitute:
Answer:

7. Given: Asked: Formula: W=Fd Substitute: Answer:
Problem 2 A lawn mower is pushed with a force of 80N. If 12,00J of work is done in mowing the lawn, what is the total distance the lawn mower was pushed?
Given:
Asked:
Formula: W=Fd
Substitute:
Answer:

8. Given: Asked: Formula: W=Fd Substitute: Answer:
Problem 3 The brakes on a car do 240,000J of work in stopping a car. If the car travels a distance of 50m while the brakes are being applied, what is the force the brakes exert on the car?
Given:
Asked:
Formula: W=Fd
Substitute:
Answer:

9. Given: Asked: Formula: W=Fd Substitute: Answer:
Problem 4 The force needed to lift an object is equal in size to the gravitational force on the object. How much work is done in lifting an object with a mass of 5.0 kg a vertical distance of 2.0 m?
Given:
Asked:
Formula: W=Fd
Substitute:
Answer:

10. Power Power is the rate of doing work. How much work is done in 1 sec
P = W/t
W (joules) joules = kg m2/s2
Unit for power is joules/second called a Watt
A Watt is kg m2/s3
Since work is energy transferred
P= E/t

11. Power Example
You push a box with a force of 100 N across the floor 5 meters it takes you 45 seconds. Your friend pushes a similar box also with a 100 N force also 5 meters across the floor but it only takes your friend 30 seconds.
How much work do you each do?
How much power do you each expend?

12. Problem 1 Page 130 In lifting a baby from a crib, 50 J of work are done. How much power is needed if the baby is lifted in 2 s?
Given:
Asked: Units?
Formula: P= W/t
Substitute:
Answer

13. Problem 2 Page 130 If a runner’s power is 130 W as she runs, how much work is done by the runner in 10 minutes?
Given:
Asked: Units?
Formula: P= W/t
Substitute:
Answer

14. Problem 3 Page 130 The power produced by an electric motor is 500 W
Problem 3 Page 130 The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?
Given:
Asked: Units?
Formula: P= W/t
Substitute:
Answer

15. Energy and Work

16. Power Practice Problems P130: 1,2,3
You do 900 J of work pushing a sofa. If it took 5s to move the sofa, how much power did you use?
Given
Asked? Units?
Formula
Substitute
Answer

17. Section Review P131: 5,6,7 Work is a transfer of energy W= Fd
(force in Newtons (kg m/s2, distance in m)
N*m or Joules
Power is the rate of doing work.
How much work is done in 1 sec
P = W/t
W (joules) joules = kg m2/s2 , t= seconds
Joules/second = Watt

18. Machines (P132) A machine is a device that makes work easier.
Simple machines (not motorized)
How machines make work easier
A machine can increase the force that is applied to an object.- Car jack (p 132) Multiplies force
A machine can make work easier by increasing the distance which reduces the force needed. Ramp (p133)
A machine can change the direction of the force- splitting wedge (p133)

19. Work Done by Machines Figure 7 Page 133
Choice – lift the chair straight up the height of the truck.
W = Fg x height
Or- slide the chair up the ramp
Work will be the same
Distance will be greater so force will be less

20. Work, Distance and Force
Machines don’t decrease the work (real machines increase the work), but they can decrease the force required Ideal machine (p135) Workin= Workout
Win = Fin *5 m ,
Wout= Win
Wout= 300J but d=5m
so F= 60N
Wout= 100 N * 3m = J
5 m
3 m
100 N
4 m

21. Factors of Work Input force – the force applied to the machine
Output force – the force applied by the machine
Input distance – the distance the input force moves
Output distance – the distance the output force moves.
work in = input force x input distance
work out = output force x output distance

22. Pulling a Nail work out = work in Figure 10 page 135
hammer claw moves 1 cm to pull a nail
Handle moves 5 cm
Output force of 1,500 N
Input force = ?

23. Ideal Machines (P135) Workin= Workout
Forcein X distancein= ForceoutX distanceout
Fin X 5cm = 1500N X 1 cm
Fin= ?
distance units cancel out if the same unit

24. Ideal Mechanical Advantage
Workin= Workout
Fin X din= FoutX dout
F out
Fin
d in
dout =
F out
Fin
d in
dout
IMA = =

25. Real Machines Work out< Workin
Some of the input work is always converted into heat by friction.
Efficiency tells us how much of the input work is converted into output work.
Workout
Workin
x 100%
< 100%
efficiency =

26. Assignment Chapter 4 Practice Problems Practice Problems P 128:1,2,3
Power Practice Problems P130: 1,2,3
Section Review P131: 5,6,7
Page 137 – Applying math 5-6-7

27. Simple Machines Section 3
Page

28. Types of Simple Machines
Simple machine does work with only one movement.
Lever
Pulley
Wheel and Axle
Inclined Plane
Compound Machines

29. Lever A bar that is free to pivot (turn) about a fixed Point
The fixed point is called the fulcrum
Input arm – distance from the input force to the fulcrum
Output arm – The distance from the output force to the fulcrum.
If output arm is shorter than the input arm then the output force is greater than the input force.

30. Types of Levers (P138-139) First-class Lever Second-class Lever
Input Force
Output Distance
Output Force
Input Distance
Second-class Lever
Output Distance
Output Force
Input Distance
Input Force
Output Distance
Third-class Lever
Output Force
Input Distance
Input Force

31. Ideal Mechanical Advantage of a Lever
Force output
Force input
IMA =
Force output x Length output=Force input X Length input
L input
L output
IMA =

32. Pulley Types (P 141-142) Fixed Pulley Movable Pulley Input Force 4N
Output Force 4N 4N
Movable Pulley 2N
Input Force 2N
Output Force 4N 4N

33. Types of Pulleys (cont)
Block and Tackle 1N 1N 1N
Input Force 1N 1N
Output Force 4N 4N

34. Ideal Mechanical Advantage of a Pulley
Force output
Force input
IMA =
IMA =
Number of Strings Lifting the Load

35. Wheel and Axle (P143) ra rw Radius of wheel IMA = Radius of axel
Input Force rw
Output Force
Radius of wheel
Radius of axel
IMA =

36. Inclined Plane 100 N Length of Slope IMA = Height of Slope
F= 100N, height = 3m W= 100 N * 3m = 300J
W= 300J but d=5m so F= 60N
5 m
3 m
100 N
Length of Slope
Height of Slope
IMA =

37. Types of Inclined Plane
Ramps, inclines, road grades
Screw
Wedge
IMA = length of slope/height of slope
Page 144

38. Compound Machines Two or more simple machines that operate together.
Can opener
Automobile
Space shuttle

39. Section 3 Review
Page 146 Applying Math

40. Assignment Practice Problems P 128:1,2,3
Power Practice Problems P130: 1,2,3
Section Review P131: 5,6,7
Page 137 – Applying math 5-6-7
Page 146 Applying Math
Practice Problem Page- Due Monday
Note Taking Worksheet- due Monday
Lab on Tuesday
Chapter 5 Review Page
1-20, Due Wednesday
Test on Chapter 5 Friday

41. Formulae F out Fin d in dout Workout Workin x 100% < 100%
F = m*a , FG= m*g unit kg m/s2 = Newton
W= F*d units N*m = Joule
P= W/t or P= E/t unit J/s = Watt (kW)
F out
Fin
d in
dout
IMA = =
Workout
Workin
x 100%
< 100%
efficiency =

42. IMA of Simple Machines Workout Workin x 100% < 100% efficiency =
Force output
Force input
All Simple Machines
MA =
> 1
Lever
L input
L output
IMA =
Length of Slope
Height of Slope
IMA =
Inclined Plane
Radius of wheel
Radius of axel
Wheel and Axle
IMA =
Pulley
IMA =
Number of Strings Lifting the Load
Workout
Workin
x 100%
< 100%
efficiency =

43. Chapter 5 Review Lever Input Arm Output Arm IMA A 25 75 B 53 42 C 36 D32 99 E 10 30

44. Problems Page 153
Given:
Asked: Units
Formula:
Substitute:
Answer

45. Friday November 7 Get out your calculators and Reference Sheets
Put away your books and notes.
Answer all the questions on the test sheets
Don’t overlook problems on the back of the test.
When you have finished, place test in the tray.
Place all homework in the tray
After the test sit quietly and begin reading chapter 6 on page 158.