Work and Machines
Chapter 5
Sections 1-3
C5- Work & Machines
Section 1- Work slides 3-13
 Section 2- Using Machines slides 14-30
 Section 3- Simple Machines slides 31-50

Section 1- Work
What You’ll Learn:
 What work is
 How work & energy are related
 How to calculate work & power

Work

Work is done when a
force causes an
object to move in the
same direction that
the force is applied.
 When a force makes
an object move,
energy is transferred
from one object to
another.
<www.mcasco.com/p1wke.html>
How do you know if you are doing
work?

Two things must happen:
– A force must be applied to the object
– The object must move in the same direction
as the force
What does direction have to do
with work?
When you pick up a stack of books, your
arms apply a force upward.
 You start walking with the books.
 The direction of motion has changed; your
arms are no longer working, instead your
legs supply the forward direction

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Work & Energy
Energy is the ability to
cause change or to do
work.
 When work is done
energy is transferred
from one object to
another.

<http://www.mnsu.edu/news/article>
Calculating Work
You multiply force times distance to
calculate work.
 Work (joules)= force (N) X distance (m)
 W=Fd
 Calculate the amount of work a painter
does when he lifts a can of paint weighing
40 newtons 2 meters.
 W=Fd=(40N) (2m)= 80 J

When is work done?
When a pitcher
throws a ball to the
catcher, he applies
force to the ball only
when it is in his hand.
 The ball moves 10 m
after it leaves his
hand but work
occurred only while it
was in his hand for
1m.

<http://www.statenews.com/media/>
Power

Power is the rate at
which work is done.
 Something has more
power if it can do the
same amount of work
in less time.
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How do you calculate power?
To calculate power, divide the amount of
work done by the time it takes to do it.
 Power (in watts)= work (in joules)/time(s)
 P=W/t The SI unit for power is the watt.
 Find the power of the machine that can do
5,000 joules of work in 20 seconds.
 P=W/t=5000J/20 s=250 watts. The
power of the machine is 250 watts.

How is power calculated when
energy is transferred?
You can also calculate power by dividing
the amount of energy transferred by the
time it took to transfer it.
 Power (in watts)=energy transferred (j)/
time (s) or P=E/t.

How is energy transferred when no
work is done?
Suppose a light bulb changes electrical
energy into light & heat at a rate of 100 j
/2 s. How many watts of power will the
light bulb have?
 P=E/t =100j/2s= 50 watts

Section 2- Using Machines
What You’ll Learn:
 How machines make work easier
 How to calculate mechanical advantage
 How to calculate the efficiency of a
machine

What is a machine?
A machine is a
device that makes
work easier.
 Knives, scissors, and
doorknobs are simple
machines.
 Engines are more
complex machines

<http://www.mshp.dps.missouri.gov/M
SHPWeb/PatrolDivisions>
Making Work Easier
There are 3 ways machines make work
easier:
 Increasing force on an object;
 Increasing the distance over which a force
is applied; or
 Changing the direction of an applied force.

How can force be increased using a
machine?
If Work= force X distance, then either
force or distance increases.
 If distance increases, then force
decreases.
 Machines such as a car jack, increase the
distance a force is applied thus reducing
the amount of force needed to do the
same amount of work.

<http://www.dynamicscience.com.au/tester/solutions/hydraulicus/gearrackandpinionc
arjack1.gif>
How does increasing distance
decrease force?
A rake makes the
work easier by
increasing the
distance over which
you apply force.
 Your hands move a
small distance at the
top of the handle
while the handle
moves across a wider
distance.

<http://themongiatkellyclan.ca/wordpre
ss/wp-content/uploads/2006/11/rakingleaves.jpg>
Why do you want a machine that
will change direction?
Sometimes it’s easier to apply force in a
different direction:
 To raise a flag up a pole, it’s easier to pull
down on a rope than to climb to the top.
 When you use an axe to split wood, a
downward force on the handle changes
into a sideways force by the blade.

The Work Done by Machines
Pushing down on the
handle raising the lid.
 You are doing work
on the crowbar that
opposes the friction of
the nails in the lid and
crate.

<http://images.jupiterimages.com/c
ommon/detail/82/80/23298082.jpg>
What are input forces & output
forces?
A machine changes
the way you do the
work making it easier.
 The force you apply
to the machine is
called the input
force or Fin.
 The force applied by
the machine is called
the output force or

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ions/nail-pulling-01.jpg>
Fout.
What are input work & output
work?

When you use a machine there are 2 kinds
of work:
– Input work or Win , done by you
– Output work or Wout, done by machine
How do machines use conservation
of energy?
When you do work on a machine, you
transfer your energy to the machine which
then transfers energy to the object.
 So, Wout is never greater than Win.
 However, a machine does not transfer all
its energy to the object. Due to friction,
some of the energy changes to heat.
 This means that Wout is always smaller
than Win.

What is an ideal machine?
If you could build perfect machine
without any friction, the input work would
equal the output work.
 Win=Wout or FinX din=FoutX dout
 If the machine could increase the input
force, then work output would be greater
than the work input.

Mechanical Advantage

Some machines make work easier by
making the output force greater than the
input force. The number of times the
applied force is increased by a machine is
called the mechanical advantage (MA) of
the machine. Mechanical advantage is
the ratio of the output force to the input
force. MA = Fout/Fin
Mechanical Advantage
Using a pulley system you only need 300
N to lift a piano that weighs 1500 N.
What is the MA?
 MA = Fout/Fin = 1500 N/300 N= 5


Notice this ratio cancels out units of
newtons.
What is ideal mechanical
advantage?
The MA of a machine
without friction is
called the ideal MA or
IMA.
 You can calculate this
by dividing the input
distance by the
output distance.

<http://www.cpo.com/images/produc
ts/em-ropesandpulleygirl.jpg>
Efficiency
Some of the energy put into a real
machine is changed into heat by friction.
 So, the output work of a machine is
always less than the work put into it.
 Efficiency is the comparison of the
amount of work put into a machine to the
amount of work the machine puts out.
 High-efficiency means less heat from
friction!

How do you calculate efficiency?
Divide the output work by the input work
to get a percentage.
 Efficiency= Wout/in X 100%
 To calculate the efficiency of a machine
with Win of 50 joules & a Wout of 40 joules:
40/50=0.8, or 80%.
 The efficiency of a real machine is always
less than 100% due to friction.

How can machines be made more
efficient?

Reducing friction by adding oil or grease to the
surfaces that rub together fills the gaps between
them so the surfaces slide across each other
more easily.
Section 3- Simple Machines
What You’ll Learn:
 Six types of simple machines
 How simple machines make work easier
 How to calculate the ideal mechanical
advantage of simple machines

Types of Simple Machines
A simple machine is a machine that
does work with only one movement of the
machine.
 There are six types: lever, pulley, wheel &
axle, inclined plane, screw, and wedge.
 The screw and wedge are different forms
of the inclined plane.

Levers
A wheelbarrow, a rake & a baseball bat
are all examples of levers.
 A lever is a bar that pivots, or turns
around, a fixed point called the fulcrum.
 The input arm is the distance from the
fulcrum to the point where the input force
is applied; the output arm is the distance
from the fulcrum to the point where the
lever exerts the output force.

What are the 3 classes of levers?

The class of a lever is based
on the location of the fulcrum,
the input force and the output
force.
<http://www.daviddarling.info/images/levers.jpg>
First-Class Lever
The top figure shows a firstclass lever with the fulcrum
located between the input and
output forces.
 The first-class lever always
changes the direction of the
force.
 Examples include a crowbar,
scissors, and a seesaw.

<http://www.daviddarling.info/images/levers.jpg>
Second-Class Lever
The middle figure represents
a second-class lever with the
output force between the
input force and the fulcrum.
 Both input and output forces
move in the same direction.
 The wheelbarrow is a classic
example.

<http://www.daviddarling.info/images/levers.jpg>
Third-Class Lever
The bottom figure, a thirdclass lever, shows the output
force is farther away from the
fulcrum than the input force.
 The output force is always
less than the input force in a
third-class lever such as a
baseball bat, but the
advantage is that it increases
the distance over which the
output force is applied.

<http://www.daviddarling.info/images/levers.jpg>
How is ideal mechanical advantage
of a lever calculated?
To calculate IMA of any machine, divide
the input distance by the output distance.
 For a lever, the input distance is the length
of the input arm & output distance is the
length of the output arm.
 IMA= Lin/Lout

Pulleys
To raise a sail upward, a
sailor pulls down on a rope
wrapped around a pulley.
 A pulley is a grooved
wheel with a rope, chain or
cable wrapped around it.
 Pulleys may be fixed or
movable or in systems.

What is a fixed pulley?
This modified firstclass lever changes
the direction of the
input force like on a
sail or a flagpole.
 An elevator also uses
a fixed pulley with a
cable.

<http://www.legoeducation.com/share
dimages/content/Large/L_Pulley_dia1_f
ixed.gif>
What is a movable pulley?
A movable pulley has
one end of the rope
fixed & the wheel is
free to move.
 The movable pulley
doesn’t change the
direction of the force,
but it does decrease
the amount of input
force needed to lift
the object. MA=2

<http://goldridge08.com/pictures/simple/actpulmov.gif>
What is a block and tackle?
A block and tackle is a 
system of fixed &
movable pulleys used
together.
 The more sections of
the rope a system
uses to pull up an
object, the greater
the output force is.
IMA=# of sections
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4560>
Wheel and Axle
Simple machine with
an axle attached to
the center of a larger
wheel and both turn
together.
 Doorknobs and ferris
wheels are examples.

<http://www.dkimages.com/discover/pr
eviews/741/131247.JPG>
What is the IMA of the wheel &
axle?
A wheel and axle is a modified lever with
the center of the axle as the fulcrum.
 To calculate the IMA of a wheel and axle,
use this equation:
IMA= radius of wheel (m)/radius of axle (m)
or IMA= rw/ra
 To increase IMA, simply increase the
radius of the wheel.

How do gears work?
A gear is a wheel and
axle with teeth
around the rim of the
wheel.
 One gear makes the
other turn with the
smaller gear turning
more times than the
larger one. Output
force & direction can
be changed with a
gear.

<http://www.coeshow.com/shop/imag
es/uploads/CRF150-Gears.jpg>
Inclined Planes
An inclined plane is
a sloping surface that
reduces the amount
of force it takes to do
work.
 Examples include
ramps and stairways.

<http://www.jaha.org/edu/inclined_pl
ane/images/IP1_000.jpg>
How does an inclined plane make
work easier?
You do the same work by lifting a box
straight up or pushing it up a ramp.
 As the inclined plane becomes longer, the
force needed to move the object becomes
less.
 The input force is applied over a longer
distance, so it takes less input force.
IMA=length of slope(m)/ height of slope(m)

The Screw
A screw is an inclined
plane wrapped in a
spiral around a post.
 The inclined plane
forms the threads on
the screw.
 Apply force by turning
the screw; friction
holds it in place.
 Examples: jar lid,
corkscrew, drill bit,
light bulb.

<http://cnx.org/content/m13594/l
atest/screw.gif>
The Wedge
A wedge is an
inclined plane with
one or two sloping
sides.
 Like the screw, the
inclined plane moves
through the object.
Knives are wedges.
 The IMA increases as
it gets longer &
thinner.

<http://www.canadianhomeworkshop.
com/stuff/photos/oct03b.jpg>
Compound Machines
Some machines, like
this can opener, are
made of several
simple machines.
 Two or more
working together are
called a compound
machine.
 The handles are
levers, a wedge
pierces the can, a
wheel & axle turns to
open the can.

<http://www.focuspg.com/itm_img/709
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