; Chapter 14 Work_ Power_ and Machines
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# Chapter 14 Work_ Power_ and Machines

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```									Work, Power, &
Machines
Chapter 14
Mr. Kovacs
Integrated Chemistry and Physics

1
What is work?

• The product of the force applied to
an object and the distance through
which that force is applied.
2
What is work?
• According to the physics
definition, you are NOT
doing work if you are
just holding the weight
• You are doing work only
while you are lifting the
• No movement : No work
3
For work to be done on an object, the
object must ___________?____________.

• move in the direction of the force.

4
Work Requires Motion
• If the wall doesn't move, the prisoner
does no work.

• No movement : No work                    5
Work Depends on Direction
• 1) Work must have a force
• 2) The force must be in the direction of
the motion

Force, F

distance, d

6
Calculating Work
• To do work on an object you have to
push the object a certain distance in
the direction that you are pushing
• Work = force x distance = F x d
• If I carry a box across the room I do
not do work on it (the box) because
the force is not in the direction of
the motion. Was any work done?
7
Is work being done or not?
Mowing the lawn        •   YES
Weight-lifting         •   YES
Carrying groceries     •   NO
Moving furniture up    •   YES
a flight of stairs
Pushing against a      • NO
locked door
Swinging a golf club   • YES
8
Is work being done or not?

•   Climbing stairs?
•   Lifting a book?
•   Pushing a shopping cart?
•   Carrying a football?
9
Calculating Work

All or part of the force
must act in the direction
of the movement.
10
Units of Work: The Joule
• 1 newton-meter
is a quantity
known as a
joule (J).
• Named after
British physicist
James Prescott
Joule.
•(1818-1889)   11
What is the SI unit of work?

Duh!!!!!
• The joule!
• Or 1 NM!

12
Using the Work Formula
• Work = Force x Distance
F = 500 pounds (2000 N)
D = 8 feet (2.5 meters)

• W = 2000 N x 2.5 m
= 5000 N-m
= 5000 J

13
Do you do more work when you
finish a job quickly?

•NO
• Work does NOT involve time, only force
and distance.

14
• How quickly work is done.
• Amount of work done per unit time.
• If two people mow two lawns of equal
size and one does the job in half the
time, who did more work?
• Same work. Different power exerted.
• POWER = WORK / TIME

15
What does power measure?

• The rate of doing work!!!!!!
• How fast the work is done!
• Work/time

16
James Watt
• A unit named after
Scottish inventor
James Watt.
• Invented the steam
engine.
• P = Work/time
– Joules/second
– 1 watt = 1 J/s

17
Calculating Power: Page 415

1.0 m

18
You row a boat across a pond. You do
3600 J of work on the oars in 60 seconds.
How much power did you use?

• 3600 J /60 sec = 60 J/sec = 60 W

19
watts
• Used to measure
power of light
bulbs and small
appliances
• An electric bill is
measured in
kW/hrs.
• 1 kilowatt = 1000 W
20
Horsepower (hp) = about 746 watts
(car,motorcycle,lawn-mower)

• The term horsepower was developed to
quantify power. A strong horse could
move a 746 N object one meter in one
second.                                  21
What is the SI unit of power?

• Watt

22
How much power does a 100 watt light
bulb use if it is turned on for 30 seconds?
• One more duh!
• 100 watts!!!!!!!!!!!!!!!!!!!

23
End of Section 1

24
Machines Do Work
• A device that makes work easier.
• A machine can change the size, the
direction, or the distance over which a
force acts.

25
Ramps are useful machines!
• It makes it easier to move.

Increasing Distance
Reduces Force

26
Increasing Force
A ramp can reduce the force

WORK DONE                       WORK DONE
big force  little distance   little force  big distance

27
Forces involved:

• Input Force • Output Force
–FI          –FO
–Force       –Force
applied to   applied by
a machine    a machine
28
Two forces, thus two types of work
• Work Input            • Work Output
 Work done on a         Work done by a
machine                 machine
=Input force x the      =Output force x the
distance through        distance through
which that force acts   which the resistance
(input distance)        moves (output
distance)

29
Figure 7 page 419

30
Can you get more work out than you
put in?

•NO
Work output can never be greater than
work input.
31
End of Section 2

32
How Does Input Force
Location Affect a Machine?
A nutcracker is a machine used to make cracking
nuts easier. As shown below, use a nutcracker to
crack three nuts, each time squeezing the
nutcracker’s handles at a different location.

33
Applying force at which handle location resulted in the
nutcracker cracking the nuts the most easily?
The nutcracker worked best when force was applied at
location 1.
How does the distance from the nutcracker’s pivot point
to the point where the force is applied affect the
nutcracker’s ability to crack nuts?
The greater the distance between the pivot and the
force, the better the nutcracker was at breaking nuts.
34
• The number of times a machine
multiplies the input force.

35
• ACTUAL
• Involves friction.
• Calculated the same for all machines
• Actual Mechanical Advantage = Output force/Input force

36
• IDEAL
• Involves no friction.
• Is calculated differently for different
machines
• Usually input distance/output distance
– Actual mechanical advantage is always less

37

38
• MA equal to one.
(output force = input force)
• Change the direction of the applied
force only.

39
• Mechanical advantage less than one
• An increase in the distance an object is
moved (do)

40
Efficiency
• Efficiency can never be greater than
100 %. Why?
• Some work is always needed to
overcome friction.
• A percentage comparison of work
output to work input.
– work output (WO) / work input (WI)

41
End of Section 3

Thank you!

42
1. The Lever
• A bar that is free to pivot, or move
about a fixed point when an input force
is applied.
• Fulcrum = the pivot point of a lever.
• There are three classes of levers based
on the positioning of the input force,
output force, and fulcrum.

43
First Class Levers
• Fulcrum is located
between the effort
and resistance.
• Makes work easier
by multiplying the
effort force AND
changing direction.

44
First Class Levers
• Work Out = Work In
• Small force applied over large distance
is the same as large force applied over
a small distance.

F   d=F   d

45
Second Class Levers
• Resistance is found
between the fulcrum
and input force.
• Makes work easier
by multiplying the
input force, but NOT
changing direction.

46
Third Class Levers
• Input force is
located between the
output force and the
fulcrum.
• Does NOT multiply
the input force, only
multiplies the
distance.
• Examples:
47
• Ideal = input arm
length/output arm
length
• input arm =
distance from input
force to the fulcrum
• output arm =
distance from output
force to the fulcrum
48

49
2. The Wheel and Axle
• A lever that rotates
in a circle.
• A combination of
two wheels of
different sizes.
• Smaller wheel is
termed the axle.

50
3. The Inclined Plane
• A slanted surface
used to raise an
object.
• Examples: ramps,
• IMA = length of
ramp/height of ramp
Can never be less
than one.

51
4. The Wedge
• An inclined plane
that moves.
• Examples: knife,
• Mechanical
increased by
sharpening it.

52
5. The Screw
• An inclined plane
wrapped around a
cylinder.
• The closer the
the mechanical
• Examples: bolts,
augers, drill bits
53
6. The Pulley
• A chain, belt , or
rope wrapped
around a wheel.
• Can either change
the direction or the
amount of effort
force
• Ex. Flag pole, blinds,
stage curtain
54
The Pulley

55
Pulley types
• FIXED                • MOVABLE
• Can only change      • Can multiply an effort
the direction of a     force, but cannot
force.                 change direction.
• MA = 1               • MA > 1

56
Page 432 Figure 19

57
• A combination of two or more simple
machines.
• Cannot get more work out of a
compound machine than is put in.

58
Assignment:
• Pages 441-442
• 1-11, 13, 14, 15, 17, 19, 22,
26, 27, 28, 29
• WB Section 4

59
14.2 Work
• 5. A woman lifts her 100-newton child up one
meter and carries her for a distance of 50
meters to the child’s bedroom. How much
work does the woman do?

100 N X 1 m =
100 N·m or 100 joules
Note: No work is done on the child when she
carries it.

60
14.2 Power
5. A horse moves a sleigh 1.00 kilometer by
applying a horizontal 2,000-newton force on
its harness for 45 minutes. What is the power
of the horse? (Hint: Change Km’s to m’s and
convert time to seconds.)

45 min = 2700 s
2000 n X 1000 m / 2700 s
740.74 watts

61
5. A machine with a mechanical advantage of
2.5 requires an input force of 120 newtons.
What output force is produced by this
machine?

2.5 = x / 120 n
X = 2.5 x 120 n

X = 300 newtons

62
• For work to be done on an object, the
object has to ____________________.

63
• Any part of a force that does not act in
the direction of an object’s motion does
no ____________________ on an
object.

64
• The SI unit of work is the __________.

65
• The rate at which work is done is called
____________________.

66
• The SI unit of power is the _________.

67
• A machine is a device that can multiply
_____________.

68

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