Work
and
Machines
Work
Work occurs when a force causes an object to
move in the direction of the force.
•You just finished reading and summarizing an entire chapter in
your science textbook. In the scientific sense, you did NO work
at all!
•Your dad just came home from a night out bowling with his
friends. He had lots of fun and did lots of work. Applying a
force to the bowling ball to make it move through a distance is
WORK.
Working Hard or Hardly Working?
Applying a force doesn’t always result in work
being done. Work is done on an object if two
things occur:
1. The object moves as a force is applied.
2. The direction of the objects motion is the same as
the direction of the force applied.
Calculating Work
Work is a force applied through a distance. The greater the distance
through which you exert a given force, the more work you do. The
greater force you exert through a given distance, the more work you do.
The amount of work (W) done in moving an object can be calculated by
multiplying the force (F) applied to the object by the distance (d) through
which the force is applied.
W=F•d
Remember, force is expressed in newtons, and the meter is
the basic SI unit for length or distance. Therefore, the unit
used to express work is the newton-meter, which is more
simply called the joule (J).
Power – How Fast Work Is Done
• Power is the rate at which work is done.
• To calculate power (P), divide the amount of
work done (W) by the time (t) it takes to do the
work.
P= W
t
The unit used to express power is joules per
second (J/s), or the watt (W).
Questions
1.
Your neighbor asks you to help push his stalled car. You
push and push, but the car doesn’t move. You are
exhausted and sweaty. Have you done any work?
2.
What is work?
3.
You lift a chair that weighs 50 N to a height of 0.5m and
carry it 10m across the room. How much work did you do
on the chair?
4.
Explain the work/power difference between driving nails
with a hammer and using a nail gun.
5.
How would it feel if you were “power” and your enemy was
“work?”
Machines – Making Work Easier
A machine is a device that helps make work easier by
changing the size or direction of a force.
Examples:
•Using a jack and tire iron to change a flat tire
•Using a screwdriver to pry the lid off a paint can
•Using a wheelbarrow to haul a load of rocks
Give 3 other examples of machines that make work easier.
Work
Work input is the work you do on a machine. You
apply a force, called the input force, to the machine
to move it through a distance.
Work output is the work done by the machine. The
machine applies a force, called the output force,
through a distance.
Work output can never be greater than work input.
Machines do NOT increase the
amount of work done.
Climbing a Hill:
If you wanted to get to point C on the illustration below you have two
options: start at A and go straight up, or start at B and walk up the
slope. Which is easier? Well, actually, they both take the same amount
of work. The only thing you have control over is if you put forth a lot of
effort over a short distance (situation A-C) or a little effort for a long
distance (situation B-C). Lets exam what work really is (at least in
scientific terms).
Machines DO NOT save work
How is it that the path from A-C takes the same amount of work as path B-C? In
A-C we have to exert a lot of force to get up the vertical side, but we don't have to
go very far. Let's suppose that the force required is 100 Newtons and the distance
is 4 meters, the amount of work done is 400 Joules:
W=F • d
W=100 N • 4 m
W= 400 J
Getting from B-C does not require near as much force as A-C, but we do have to
move a considerably longer distance. In fact, the actual force and distance are 25
Newtons and 16 meters. Calculating the work gives:
W= 25 N • 16 m
W= 400 J
The Force-Distance Trade-off
When a machine changes the
size of the force, the distance
through which the force is
exerted must also change.
Force or distance can increase,
but not together.
When one increases, the other
must decrease.
The work output is never
greater than the work input.
Mechanical Advantage
Some machines can increase force more than
others. A machine’s mechanical advantage tells you
how many times the machine multiplies force. Use
the following equation to find mechanical advantage:
output force
MA = input force
Mechanical Efficiency
The less work a machine has to do to overcome
friction, the more efficient it is. Mechanical efficiency
is a comparison of a machine’s work output with the
work input. Use the following equation:
output force
 100
Mechanical Efficiency =
input force
The 100 in the equation means that mechanical
efficiency is expressed as a percentage.
If a machine could be made that had 100 percent
mechanical efficiency, it would be called an ideal
machine.
Do the Math…
1. You apply 200 N to a machine, and the machine
applies 2,000 N to an object. What is the formula
needed? What is the mechanical advantage?
2. You apply 10 N to a machine, and the machine
applies 10 N to another object. What is the
mechanical advantage? Can such a machine be
useful?
3. Which of the following makes work easier to do?
a. A machine with a mechanical advantage of 15
b. A machine to which you apply 15 N and that
exerts 255 N
Questions
1. How does getting regular oil changes improve the
mechanical efficiency of a car’s engine?
2. Complete this sentence: Work is being done when…
3. Why is the work output for a machine always less
than the work input?
4. You and a friend together apply a force of 1,000 N to
a 3,000 N automobile to make it roll 10m in 1 minute
and 40 seconds. How much work did you and your
friend do together? What was your combined power?
5. Mechanical Advantage is to cents as mechanical
efficiency is to ____________.
Types of Machines
1. Lever
a. First Class Lever
b. Second Class Lever
c. Third Class Lever
2. Inclined Plane
3. Wedge
4. Screw
5. Wheel and Axle
6. Pulley
Levers
A lever is a simple machine consisting of a bar that pivots at a
fixed point, called a fulcrum. Levers are used to apply force
to a load. The three classes of levers are based on the
locations of the fulcrum, the load, and the input force.
•With a first class lever, the fulcrum is between the
input force and the load.
•1st class levers change the direction of the input
force.
•Depending on the location of the fulcrum, 1st class
levers can be used to increase force or increase
distance.
•When the fulcrum is closer to the load than to the input force, the
mechanical advantage is greater than 1.
•When the fulcrum is exactly in the middle, the MA is exactly 1.
•When the fulcrum is closer to the input force than to the load, the MA is
less than 1.
•With a second class lever, the load is between the
fulcrum and the input force.
•2nd class levers allow you to apply less force than the
force exerted by the load.
•Because the output force is greater than the input
force, you must exert the input force over a greater
distance.
•A 2nd class lever has a mechanical advantage of
greater than 1.
• With a third class lever, the input force is between
the fulcrum and the load.
• 3rd class levers do not change the direction of the
input force.
• 3rd class levers do not increase the input force.
• The output force is always less than the input
force.
• They increase the distance through which the
output force is exerted.
• A 3rd class lever has a mechanical advantage of
less than 1.
Questions
1.
2.
3.
4.
5.
A third class lever has a mechanical advantage of less
than 1. Explain why it is useful for some tasks.
If you could choose to be a 2nd class lever, what would
you be? What would your job be? How does that
feel?
How is a hammer both a first class lever and a third
class lever?
True or False? A large stick used to move a heavy
rock is a simple machine.
Classify each of the following as 1st, 2nd, or 3rd class
levers: garden shovel, wheelbarrow, seesaw, broom,
baseball bat.
Inclined Planes
• An inclined plane is a simple machine that is a straight
slanted surface – a ramp.
• The mechanical advantage of an inclined plane can be
calculated by dividing the length of the inclined plane by
the height to which the load is lifted.
• An inclined plane allows you to apply a smaller force
over a greater distance.
Wedges
• A wedge is a double-inclined plane that
moves.
• A wedge applies an output force that is
greater than your input force.
• The greater the distance you move the
wedge, the greater the force it applies on the
object.
• The mechanical advantage of a wedge is
determined by dividing the length of the
wedge by its greatest thickness.
Screws
• A screw is an inclined plane that is wrapped in a spiral.
• When rotated, a small force is applied over the long distance along
the inclined plane of the screw.
• The screw applies a large force through the short distance it is
pushed.
• Screws are most commonly used as fasteners.
• The longer the spiral on a screw is and the closer together the
threads, the greater the screw’s mechanical advantage.
Archimedes’ Hydraulic Screw
Wheel and Axle
• The wheel and axle is a simple machine consisting of two circular
objects of different sizes.
• The axle is the smaller of the two circular objects.
• As the wheel turns, so does the axle. Because the axle is smaller
than the wheel, it rotates through a smaller distance, which makes
the output force larger than the input force.
• The mechanical advantage of the wheel and axle can be determined
by dividing the radius of the wheel by the radius of the axle.
Pulleys
• A pulley is a simple machine consisting of a grooved
wheel that holds a rope or cable.
• A load is attached to one end of the rope, and an input
force is applied to the other end.
• There are two kinds of pulleys: fixed and movable.
Fixed
Movable
Fixed Pulleys
•This is a fixed pulley. It doesn't move when the
rope is pulled. It is fixed to the upper bar.
•You can pull down on the rope in order to lift
the load up.
•A fixed pulley only spins. Therefore, there’s a
mechanical advantage of 1.
Movable Pulleys
•This is a movable pulley. As the rope is pulled up,
it can also move up. The weight is attached to this
moveable pulley. Each side of the rope is
supporting the weight, so each side carries only
half the weight.
•The force needed to hold up the pulley in this
example is 1/2 the weight!
•The mechanical advantage of this system is 2. It
is the weight (output force) divided by 1/2 the
weight (input force).
More on Pulleys…
• When a fixed pulley and a movable pulley
are used together, the pulley system is
called a block and tackle.
You can see that the weight is now
suspended by two pulleys rather
than one. That means the weight is
split equally between the two
pulleys, so each one holds only
half the weight, or 50 pounds (22.7
kilograms). That means that if you
want to hold the weight suspended
in the air, you only have to apply
50 pounds of force (the ceiling
exerts the other 50 pounds of
force on the other end of the rope).
Questions
1.
Why is a set of stairs classified as an inclined plane?
2.
Name and classify at least four simple machines found
in a kitchen.
3.
Pretend you are a simple machine reporter for the
“Machines’ Daily News”. Write a short fictional account
of your travels through at least two work related
scenarios.
4.
Assume there is a hit television show titled “Machine
Shop.” What are the titles of five main characters that
might star on the show?
(For example, Handy McAnnic might be the leading character.)
Compound Machines
Compound machines are made of two or more
simple machines.
Can you identify simple machines within a compound
machine?
Answers…
Item Name
Types of simple machines that
make it up
Can opener
Inclined plane, wedge, wheel and axle
Pencil sharpener
Inclined plane, screw, wedge, wheel and axle
Scissors
Inclined plane, wedge, first class lever
Stapler
Inclined plane, wedge, second class lever
Bicycle
Wheel and axle, levers, pulleys, screws
Block and Tackle
2 or more pulleys
Mechanical Efficiency of Compound Machines
In general, the more moving parts a machine has,
the lower its mechanical efficiency.
Most compound machines have low mechanical
efficiency.
An automobile is an example of a compound
machine that involves MANY simple machines. Too
much friction could cause heating and damage to the
simple machines involved. It’s important that friction
be reduced through use of lubrication.
Questions
1. You are one simple machine and your best friend is another
simple machine. What are each of you and how do you
work together?
2. The radius of the wheel of a wheel and axle is 4 times
greater than the radius of the axle. What is the mechanical
advantage of this machine?
3. A winding road is actually a series of inclined planes.
Describe how a winding road makes it easier for vehicles to
travel uphill.
4. What compound machine would you choose to be in order
to assist hurricane victims? Explain.
5. Why do you think you would not want to reduce the friction
involved in using a winding road?
Can you identify simple machines within a compound machine?
1. A zipper is made from ________________________.
A. 2 inclined planes
B. 3 wedges
C. 3 levers
D. 4 screws
2. Identify the simple machines that make up a pair of scissors.
A. 2nd class levers and wedges
B. Inclined planes and wedges
C. 1st class levers and wedges
D. A wheel and axle and a pulley
3. Identify the simple machines that make up a manual can opener.
A. 3rd class lever, inclined plane, and wedge
B. 2nd class lever, screw, and pulley
C. Inclined plane, wheel and axle, and fixed pulley
D. Inclined plane, wedge, wheel and axle
What is mechanical advantage?
1.
Which knife gives you a better mechanical advantage?
A. a short and fat knife
B. a short and thin knife
C. a long and fat knife
D. a long and thin knife
2.
Which of the following machines has the greatest mechanical advantage?
A. a machine to which you apply a force of 50 N and the machine applies a force of 150 N
B. a machine to which you apply a force of 60 N and the machine applies a force of 200 N
C. a machine to which you apply a force of 25 N and the machine applies a force of 100 N
D. a machine to which you apply a force of 40 N and the machine applies a force of 125 N
3.
In 1994, a 3,000 kg pancake was cooked, and flipped, in Manchester, England. Suppose you had a
giant spatula that you could use as a lever to flip this pancake. Where could you best position the
fulcrum and the pancake in order to get the greatest mechanical advantage?
A. place the fulcrum close to the pancake, as in a first-class lever
B. place the pancake between the input force and the load, as in a second-class lever
C. position the input force between the fulcrum and the pancake, as in a third-class lever
D. all of these machines have equal mechanical advantage
How can you calculate the mechanical
advantage of a machine?
1. You apply 200 N to a machine and the machine applies 2,000 N to an object.
What is the mechanical advantage?
A. 0.10
B. 10
C. 1,800
D. 400,000
2. What is the mechanical advantage of an inclined plane 3m long and 0.5m high?
A. 0.5
B. 1.5
C. 3
D. 6
3. What is the mechanical advantage of a wheel and axle where the wheel’s radius
is 10 cm and the axle’s radius is 2 cm?
A. 2
B. 5
C. 10
D. 20
Who is Rube Goldberg?
•
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•
•
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•
•
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Reuben Lucius Goldberg
1883-1970
Born in San Francisco
Graduated with engineering degree from University of California Berkeley
Worked as an engineer for City of San Francisco Water and Sewer Department
Convinced his father he wanted to work as an artist
Got a job as an office boy in sports department of a San Francisco newspaper
Kept submitting cartoons until he was published
Moved to New York to draw daily cartoons for Evening Mail
Founding member of National Cartoonist Society
Pulitzer Prize winner
National figure, often-quoted television and radio personality
60-year career
Now, time for a little comic relief…
Invention for Opening the Garage Door
Without Getting Out of the Car
How to Keep Shop Windows Clean
Passing man (A) slips on banana peel (B) causing him to fall on rake (C). As handle of rake rises it throws
horseshoe (D) onto rope (E) which sags, thereby tilting sprinkling can (F). Water (G) saturates mop (H).
Pickle terrier (I) thinks it is raining, gets up to run into house and upsets sign (J) throwing it against nontipping cigar ash receiver (K) which causes it to swing back and forth and swish the mop against window
pane, wiping it clean. If man breaks his neck by fall move away before cop arrives.
Rube Goldberg - extremely complicated inventions used to perform an ordinary, simple tasks
More Rube Goldberg…
Can you explain this one?
What’s happening here?
So, what?
•Rube Goldberg machines are examples of complex machines.
•All complex machines are made up of combinations of simple machines.
•Rube Goldberg machines are usually a complicated combination of simple
machines.
•By studying the components of Rube Goldberg machines, we learn more
about simple machines.
•The cartoons symbolize “man’s capacity for exerting maximum effort to
accomplish minimal results.”
•The cartoons depict convoluted machines functioning in complex ways to
perform simple tasks.
•“Rube Goldberg” has become synonymous with any complex system achieving
a basic task.
Next week you’ll create your own “Rube Goldberg” machine!
Questions
1.
How are machines an important part of our lives?
2.
How might “machines” be important to animal
survival?
3.
How would your life change if inclined planes no
longer existed?
4.
Write a narrative poem about machines, work,
power, and force.
Websites
http://staff.harrisonburg.k12.va.us/~mwampole/1-resources/simple-machines/index.html
http://www.beaconlearningcenter.com/Weblessons/SimpleMachines/machines002.htm
http://www.generalpatton.org/education/lesson_plans/PM_lessonPlan_sm.pdf
http://juniorengineering.usu.edu/workshops/machines/machines.php
http://discover.edventures.com/functions/termlib.php?action=&termid=67&alpha=t&searchString=
http://infao5501.ag5.mpi-sb.mpg.de:8080/topx/archive?link=Wikipedia-Lip6-2/38857.xml&style
http://en.wikipedia.org/wiki/Hammer
http://www.usoe.k12.ut.us/curr/science/sciber00/8th/machines/sciber/lever3.htm
http://www.enchantedlearning.com/physics/machines/Levers.shtml
http://www.usoe.k12.ut.us/curr/science/sciber00/8th/machines/sciber/machine2.htm
http://www.americanwheelchairs.com/ramps.shtml
http://www.rollaramp.co.uk/accessories.htm
http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=doorstop%20picture
http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=plow%20picture
http://search.msn.com/images/results.aspx?q=axe+head&mkt=en-us
http://en.wikipedia.org/wiki/Screw/Bolt
http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=doorknob%20picture
http://search.msn.com/results.asp?FORM=sCPN&RS=CHECKED&un=doc&v=1&q=%22ferris%20wheel%22%20picture
http://search.msn.com/images/results.aspx?q=%22steering+wheel%22&mkt=en-us
http://www.howstuffworks.com/pulley.htm
Bibliography
Silver, H. F., Hanson, J. R., Strong, R. W., Schwartz, P. B. Teaching Styles and Strategies: Interventions to Enrich
Instructional Decision Making. Trenton: Thoughtful Education Press, 1996.
Holt Science & Technology: Physical Science. Austin: Holt, Rinehart and Winston, 2001.