1. Work, Power, and Machines
Chapter 14

2. Chapter Pretest- Assess your Prior Knowledge
According to Newton’s first law, if no net force acts on an object, the object continues in motion with constant ________.
Velocity
Force
Acceleration A

3. Chapter Pretest- Assess your Prior Knowledge
A horizontal force on an object can be broken down into these components:
5N north
5N east
If no other forces act on the object, in what direction will the object move?
Northeast

4. Chapter Pretest- Assess your Prior Knowledge
Newton’s second law of motion states that the net force acting on an object equals the product of what two variables?
Mass
Acceleration

5. Chapter Pretest- Assess your Prior Knowledge
A machine produces an output force of 12.3N when an input force of 8.6N is applied.
What is the ratio of the machine’s output force to its input force?
1.4

6. Chapter Pretest- Assess your Prior Knowledge
A person exerts 22N on a box. If a frictional force of 3N opposes this force, what is the net force acting on the box?
19N

7. Chapter Pretest- Assess your Prior Knowledge
A machine has an output force of 57.3N when a force of 32.6N is used to operate the machine.
What is the percentage increase in the force?
176%
Output/Input

8. Chapter Pretest- Assess your Prior Knowledge
A small wheel has a radius of 32cm, and a large wheel has a diameter of 128cm.
What is the ratio of the diameters of the large wheel to the small wheel? 4 2
0.25
0.5 B

9. 14.1 Work and Power
I will describe the conditions that must exist for a force to do work on an object
I will calculate the work done on an object
I will describe and calculate power
I will compare the units of watts and horsepower as they relate to power

10. What is Work? Work When is work done?
The product of force and distance
When a force acts on an object in the direction the object moves
Example:
Work is done by the boy in the green hat when he exerts a horizontal force to push the cart down the road

11. Work Requires Motion NO movement NO Work is Done
For a Force to Work on an Object
NO movement NO Work is Done
Some of the force must act in the same direction as the object moves
Example:
Weightlifter
WORK done when he exerts an upward force to raise the barbell over his head
NO WORK done as he stands with barbell over his head (NO MOVEMENT!)

12. Work Depends on Direction
Direction of force
Direction of motion
Work Depends on:
When force and motion are in the same direction the work done is MAXIMIZED!
ANY part of a force that does NOT act in the direction of motion does NO WORK on an object

13. A C B Demonstrating Work FORCE A Force + Motion = same direction
WORK MAXIMIZED! B
ONLY horizontal part of the force does work to move suitcase to the right C
Lifting force NOT in the direction
of motion therefore the force
does NO work on the
suitcase C
FORCE
DIRECTION OF MOTION
This force does work B
Force
FORCE
This force does NO work

14. Press Object Against Wall
DEMO
Drop Object
Force = gravity (downward)
Force + motion = same direction
So, WORK DONE
Press Object Against Wall
Force = person pushing object towards wall
NO motion
NO WORK
Push Object Across Desk
Forces = gravity (downward) + person pushing (horizontally)
Motion = horizontal
WORK DONE HORIZONTALLY ONLY
Describe:
The force acting on the object
The motion of the object
Is work being done on the object?
Drop Object
Press Object Against Wall
Push Object Across Desk

15. Using the Work Formula Work = Force X Distance J = N X m
Calculating Work
Units of Work
Joule (J) = N*m
When a force of 1 N moves an object 1 m in the direction of the force, 1 J of work is done
Using the Work Formula
Work = Force X Distance
J = N X m
Wdone = 1600 N x 2.0 m
Wdone = 3200 N*m = 3200 J

16. YOUR TURN:
How much work does a 25-Newton force do to lift a potted plant from the floor to a shelf 1.5 meters high?
F= 25 N
d = 1.5 m
W done = ?
Wdone = F x d
Wdone = 25 N x 1.5 m
Wdone = 37.5 N*m = 37.5 J

17. What is Power? Power Doing work at a faster rate requires MORE power
The rate of doing work
Doing work at a faster rate requires MORE power
To increase power:
Increase the amount of work done in a given time OR
Do a given amount of work in less time

18. Power Example-Snow Removal
Person shoveling
Person doing work
Slower time
Less power
Snow blower
Machine doing work
Faster time
More power

19. Using the Power Formula
Calculating Power
Units of Power
Watts (W)= N*m/s = J/s
Equal to one Joule per second
Example
40 watt light bulb
Requires 40 J per second it is lit
Using the Power Formula
Power = Work / Time
W = J / s
W= (72 N x 1.0 m) /2.0 s
W= 36 Nm/s = 36 J/s = 36 W

20. Your Turn:
Your family is moving to a new apartment. While lifting a box 1.5m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?
F = 200 N
t = 1.0 s
d = 1.5 m
Power = ?
Power = work / time
Power = ( F x d) / t
Power = (200 N x 1.5 m) / 1.0s
Power = 300 N*m/s = 300 J/s = 300 W

21. Ticket In
You lift a book from the floor to a bookshelf 1.0m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s?
You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used?

22. Work & Power -Extra Credit Must show all work, NO TALKING
A student rows a boat across a still ponds, doing 3600 J of work on the oars in 60 s. What is the student’s power output?
A truck pulls a trailer at a constant velocity for 100 m while exerting a force of 480 N for 1 minute (60 s). Calculate the power.
(hint: 1st calculate the work!)

23. Horsepower (hp) 1 horsepower (hp) = 746 Watts (W)
Compared the power outputs of steam engines to the power output of a very strong horse.
Example:
Rate of 4 horsepower
HORSE DRAWN PLOW
GASLOINE POWERED
ENGINE (SNOWBLOWER)

24. Work & Machines Machine Device that CHANGES a force Example
Car jack
You apply force to jack handle
Jack CHANGES this force and applies a MUCH GREATER force to the car
Lug Wrench
You apply force to wrench handle
Jack CHANGES this force and applies a MUCH GREATER force to the lug nut
Make work EASIER to do
CHANGES
SIZE of force needed
DIRECTION of a force
DISTANCE over which a force acts

25. Increasing Force
When a machine INCREASES the distance over which you exert a force…then it DECREASES the amount of force you need to exert
Example: Car Jack
Each rotation of the jack handle applies a small force over a long distance
Each rotation lifts the car only a short distance

26. Increasing Distance
When a machine DECREASES applied force, but INCREASES the distance over which the force is exerted
Example: Boat Oars
Act as machines that increase the distance over which the force acts

27. Changing Direction
When a machine CHANGES the direction of the applied force
Example: handle of oar
Pull back on handle
The other end moves in opposite direction

28. Work Input and Output
Because of friction, the WORK done by a machine is ALWAYS LESS then the work done on the machine

29. Work Input to a Machine Input force Input distance Work input
The force you exert on a machine
Input distance
The distance the input force acts through
Work input
The work done by the input force acting through the input distance
Work
Input
Input Force
Input Distance

30. Work Output on a Machine
Output Force
The force exerted by a machine
Output Distance
The distance the output force exerted through
Work Output
The work done by the output force acting through the output distance
Work
Output
Output Force
Output Distance

31. Mechanical Advantage MA of a Machine Example: nutcracker
The number of times that a machine increases an input force
Example: nutcracker
A: Force = 7 times greater than the force you exert on the nut cracker
MA = 7
B: Force = 3 times greater than the force you exert on the nut cracker
MA = 3 A B

32. Actual Mechanical Advantage
AMA
The mechanical advantage determined by measuring the actual forces acting on a machine
Equals the ratio of the output force to the input force
Example:
MA of rough surface < smooth surface
Greater force is needed to overcome friction
Output
Force
AMA
Input force

33. Ideal Mechanical Advantage
IMA
The mechanical advantage in the absence of friction
Because friction is ALWAYS present, the AMA of a machine is always LESS than the IMA
Because friction reduces MA, engineers often design machines that
Uses low-friction materials
Use lubricants

34. Calculating Mechanical Advantage
Ideal Mechanical Advantage (IMA)
Easier to calculate than AMA
Neglects effects of friction
Input Distance
IMA
Output distance

35. Efficiency Work output always LESS than work input Efficiency
Because machine must overcome friction
Efficiency
The % of work input that becomes work output
Always LESS than 100%
Because always some friction
REDUCE FRICTION
= IMPROVED EFFICIENCY
Work Output
Efficiency
Work Input
X 100%