Work, Power,
& Machines
What is work ?
 The product of the force applied to an
object and the distance through which
that force is applied.
Is work being done or not?
 Mowing the lawn
 Weight-lifting
 Moving furniture up a
flight of stairs
 Pushing against a
locked door
 Swinging a golf club
 YES
 YES
 YES
 NO
 YES
Calculating Work
All or part of the force must
act in the direction of the
movement.
Do you do more work
when you finish a job
quickly?
 Work does NOT involve time, only force
and distance.
 No work is done when you stand in place
holding an object.
 Labeling work: w = F x d
 Newton X meter (N m)
 Which also = (kg x m2)
s2
The Joule
 1 newton-meter
is a quantity
known as a
joule (J).
 Named after
British physicist
James Prescott
Joule.
 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
The watt
 A unit named after
Scottish inventor
James Watt.
 Invented the steam
engine.
 P = W/t
 Joules/second
 1 watt = 1 J/s
watts
 Used to measure
power of light
bulbs and small
appliances
 An electric bill is
measured in
kW/hrs.
 1 kilowatt = 1000 W
Horsepower (hp) = 745.5
watts
 Traditionally associated with engines.
(car,motorcycle,lawn-mower)
 The term horsepower was developed to
quantify power. A strong horse could
move a 750 N object one meter in one
second.
750 N
Machines
 A device that makes work easier.
 A machine can change the size, the
direction, or the distance over which a
force acts.
Forces involved:
 Input Force  Output Force
FO
FI
Force
Force
applied
by
applied to
a machine
a machine
Two forces, thus two
types of work
 Work Input
 work done on a
machine
=Input force x the
distance through
which that force acts
(input distance)
 Work Output
 Work done by a
machine
=Output force x the
distance through
which the resistance
moves (output
distance)
Can you get more work
out than you put in?
Work output can never be greater than
work input.
Mechanical Advantage (MA) –
expressed in a ratio WITH
NO UNITS!!
 The number of times a machine
multiplies the input force.
2 types of mechanical
advantage
 IDEAL
 Involves no
friction.
 Is calculated
differently for
different machines
 Usually input
distance/output
distance
 ACTUAL
 Involves friction.
 Calculated the
same for all
machines
Different mechanical
advantages:
 MA equal to one.
(output force = input
force)
 Change the direction
of the applied force
only.
 Mechanical
advantage less than
one
 An increase in the
distance an object is
moved (do)
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)
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 effort force,
resistance force, and fulcrum.
First Class Levers
 Fulcrum is located
between the effort
and resistance.
 Makes work easier
by multiplying the
effort force AND
changing direction.
 Examples:
Second Class
Levers
 Resistance is found
between the fulcrum
and effort force.
 Makes work easier
by multiplying the
effort force, but NOT
changing direction.
 Examples:
Third Class Levers
 Effort force is located
between the
resistance force and
the fulcrum.
 Does NOT multiply
the effort force, only
multiplies the
distance.
 Examples:
Levers!!!!!!!!!!!
Mechanical advantage of
levers.
 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
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.
 IMA = radius of
wheel/radius of axle.
3. The Inclined
Plane
 A slanted surface
used to raise an
object.
 Examples: ramps,
stairs, ladders
 IMA = length of
ramp/height of ramp
Can never be less
than one.
4. The Wedge
 An inclined plane
that moves.
 Examples: knife, axe,
razor blade
 Mechanical
advantage is
increased by
sharpening it.
5. The Screw
 An inclined plane
wrapped around a
cylinder.
 The closer the
threads, the greater
the mechanical
advantage
 Examples: bolts,
augers, drill bits
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
Pulley types
 FIXED
 Can only change
the direction of a
force.
 MA = 1
 MOVABLE
 Can multiply an
effort force, but
cannot change
direction.
 MA > 1
MA = Count # of ropes that
apply an upward force (note
the block and tackle!)
Fe
 A combination of two or more simple
machines.
 Cannot get more work out of a compound
machine than is put in.
A horse pulls a plow 2.5
m. If the horse exerts 10
N calculate the work
done.
 Given
 Solution
A horse pulls a plow 2.5
m. If the horse exerts 10
N calculate the work
done.
 Given
F= 10 N
D= 2.5 m




Solution
W=Fd
W=(10)(2.5)
W= 20 J
A machine does 200 J of
work while moving a load
25 m. Calculate the force
exerted.
 Given
 Solution
A machine does 200 J of
work while moving a load
25 m. Calculate the force
exerted.
 Given
 W= 200 J
 D= 25 m




Solution
W=Fd
F=W/d
F= 200/25= 8 N
Bubba can shovel 12 N of
dirt a distance of 2.5 m in
25 s. Calculate his power.
 Given
 Solution
Bubba can shovel 12 N of
dirt a distance of 2.5 m in
25 s. Calculate his power.
 Given
T=25 s
D= 2.5 m
F= 12 N




Solution
P=FD/t
P= (12)(2.5)/25
P=1.2 W
1. How much work is done to move a
box if the force applied to that box
is 65.7 N and it is moved a distance
of 12.1 m?
 Given
 F= 65.7
 D=12.1




Solution
W=Fd
W= (65.7)(12.1)
W= 795 J
2. If 75 J of work are used to move a
motorcycle 3.64 m, what is the
amount of force applied to that
motorcycle?
 Given
 W= 75
 D= 3.64 m





Solution
W=Fd
F=w/d
F= 75/3.64
F= 20.6 N
3. If I have used 0.37 N on a marble
and it has a work amount of 0.16 J,
what is the distance that marble has
gone?
 Given
 W= .16 J
 F- .37 N





Solution
W=Fd
D=W/f
D= .16/.37
D= .43 m
4. A skateboarder does 438.84 J of
work. If this occurs over a period of
21.8 seconds, how much power does
that skateboarder use?
 Given
 W= 438.84
 T= 21.8 s




Solution
P=w/t
P= 438.84/21.8 s
P= 20 W
5. A construction worker uses a running
saw that weighs 35.28 N on a board that
is 3.6 m long. If the power output of the
saw is 6.35 W, what was the amount of
time the saw was used?




Given
P= 6.35 W
F= 35.28
D= 3.6 m





Solution
P=Fd/t
T=FD/P
T= 35.28(3.6)/6.35
T= 20 s
6. How much work is
applied to your backpack
if 11W of power are used
for 681 seconds?
 Given
 P= 11 W
 T=681 s





Solution
P=W/t
W=Pt
W= 11(681)
W=7491 J
 Given
 Solution
7a. A soccer ball weighs 17.3N and can
be kicked a distance of 7.5 m. What is the
work applied to the soccer ball?
b. If the ball travels that distance in 2.4
seconds, what is the soccer player’s
power?
 W=Fd
 W= (17.3)(7.5)
 W= 130 J
 P=W/t
 P= 130/2.4
 P= 52 W
8a. If a car’s motor weighs 12289.7 N and
works at 530893.44 J, what is the
distance the car has been moved?
b. Using the above number for work if the
car is used for 15 minutes (!!!), how much
power has the car used?
 W=Fd
 530893.44 =
(12289.7)d
 D= 43 m
 P=W/t
 P= 530893.44
/900
 P= 589 W
 Given
 Solution
 Given
 Solution
 Given
 Solution
 Given
 Solution