WORK
reflect
You may be reading this as part of your homework. Do you think reading is work? No
matter how difficult the reading is, it is not work in the scientific sense of the word. Of
course, you are using your muscles to lift and hold the paper and to move your pen, which
is work. However, reading words and thinking about what you have read is not work in the
same way that lifting the paper or moving a pen are work. Scientists sometimes call this
kind of work mechanical work.
What are some differences between doing your homework and doing mechanical work?
How do you think scientists measure and calculate work?
Work is done when a force moves an object.
Reading this companion is not work because you are not using forces to move objects.
(Recall that a force is a push or pull.) Whenever work is done on an object, the object’s
energy changes. Several different forms of energy can change because of work.
If a force causes an object to gain speed, then its kinetic energy has increased. Kinetic
energy is energy of motion. Consider a car accelerating from a standstill to 60 miles per
hour (mph). While the car accelerates, the engine is doing work by applying force that
causes the car to gain kinetic energy.
You can also do work by increasing an object’s potential energy.
Potential energy is stored energy. An object gains potential
energy when it is raised to a higher elevation. As long as an
object is up at some height, is has the potential to release
energy or do work as it falls back down. What force causes an
object to fall down? The force of gravity, which is equal to the
weight of the object.
gravity: the force
that attracts an object
with mass to another
object with mass
(such as Earth)
Picture a 20-pound watermelon resting on the
kitchen table. You grasp the melon, carry it outside,
walk around the block, and return the melon to the
table. Now imagine you carry the melon upstairs
to the second floor. The walk around the block
would probably seem harder because you carry the
watermelon a greater distance. However, carrying
the melon upstairs produces more work. Why?
During your walk around the block, the melon
remained the same height above the ground.
Therefore, it gained no potential energy as you
walked.
Holding a watermelon above the
ground increases its potential
energy. If you were to drop the
watermelon, its potential energy
would change to kinetic energy
as it fell.
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In comparison, the watermelon gained a lot of potential energy when you raised it from the
first floor to the second floor. The more an object’s energy changes, the more work has
been done to it.
Everyday Life: Plants at Work
Even plants have to work sometimes. Plant
cells have strong cell walls. When plants need
to, they can make their cells absorb extra
water. The extra water puts pressure on the
plant’s cell walls, which makes the cells rigid.
This keeps the plant erect. When plants don’t
get enough water, they wilt and collapse. If
more water becomes available, the plant’s
cells will swell and the plant will raise itself
up, which is work. This force that plants can
apply is called turgor force. Some plants can
exert so much of this force they can crack
pavement and split rocks!
A plant can exert enough force to break
pavement.
We can measure and calculate work.
Work equals force times distance. You can write the work formula like this:
W = Fd
Scientists measure force in units called newtons (N). (One newton equals about ¼ pound.)
Scientists measure distance in units of meters (m). So, work is measured in units called
newton-meters (N·m). Another unit for measuring work is the joule (J). One joule equals
one newton times one meter, or one newton-meter.
Suppose you used a force of 100 N to lift a watermelon a distance of 5 m. How much work
did you do? Use the work formula:
W = Fd = (100 N)(5 m) = 500 N·m = 500 J
You did 500 J of work on the watermelon. Scientists also use joules to measure energy.
So, you can also say the watermelon gained 500 J of energy.
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look out!
If a force is applied to an object but the object does not move, no work is done on the
object. Consider these two gymnasts: The woman is doing pull-ups, and the man is holding
a pose without moving. Both actions require a lot of strength, but only the woman is
moving, so only the woman is doing work!
The woman is doing work because the force is producing motion. The man is
doing no work because the force is producing no motion.
Each pull-up produces work equal to the woman’s weight (force) times the length of her
arms (distance).
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WORK
We can use simple machines to help us do work.
Simple machines are devices that make work easier and have few or no moving parts.
These machines are usually classified into six types, shown below.
Inclined Plane (example: ramp)
Wheel and axle
Wedge (example: axe head)
Lever (example: crowbar)
Screw
Pulley
Simple machines don’t do work for you. They don’t even reduce the amount of work. They
simply make work easier. Recall that work equals force times distance. If force increases,
distance must decrease; if force decreases, distance must increase. Most simple machines
help us by decreasing the applied force needed to do work. However, the force must be
applied over a longer distance.
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For example, a loading ramp is a type of inclined plane that is useful for raising a heavy
load. Suppose we want to move a heavy piano 1 m off the ground and into a truck bed.
Lifting the piano 1 m straight up would require a huge amount of force. A ramp allows us to
push the piano gradually off the ground, using a much smaller force. However, we have to
push the piano a greater distance to get it up the ramp and into the truck.
Another example of a simple
machine is a claw hammer.
We could not pull a nail out
of a board with our fingers—
the amount of force needed
is too great. However, we
can use the claws of the
hammer to pull out the nail
easily. A claw hammer is a
type of lever. It decreases
the force needed to remove
the nail. However, we must
move the hammer over a
greater distance.
An inclined plane such as a ramp (left) makes work
easier by reducing the force needed to lift a load off the
ground. A lever such as a claw hammer (right) reduces
the force needed to remove a nail from a board.
Getting Technical:
Compound Machines
Machines that are made up of several
simple machines are called compound
machines. A bicycle is a combination of
pulleys, levers, screws, and wheels and
axles. A water screw consists of a screw
placed inside a wheel and axle. This
machine has been used for thousands of
years to lift water from rivers and move it
over distances.
This ancient compound machine is a
type of water screw. Water is trapped in
the lower compartments of the machine.
Turning the screw moves the water from
lower to higher compartments.
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what do you think?
What are some other simple or compound machines you use every day? Think of the tools
you use to cut or sharpen things. Think of the tools you use to lift things off the ground or
move things over a distance.
What do you know?
Each row includes two related tasks, task A and task B. Does one task do more work, or is
the amount of work the same between task A and task B? Write your answers in the third
column.
Task A
Lifting a book from the floor
to the top of a bookshelf
Running one mile around
a track
Carrying a heavy box up a
ramp to the bed of a truck
Task B
Carrying the book from one
end of the room to the other
Running half-a-mile uphill
Which does more work?
Lifting the box off the ground
into the truck bed
connecting with your child
Analyzing Everyday Machines
With your child, explore the relationship between
machines and work by identifying different machines
used in and around your home. Many of the tools that
we use in our everyday lives are compound machines
rather than simple ones. Remind your child that a
compound machine is made up of simple machines.
An easy example is a pair of scissors. Scissors combine
wedges and levers to create a tool that cuts more easily
than either a wedge or a lever by itself. Instruct your
child to compare cutting a paper without scissors to
cutting a paper with scissors: How does this compound
machine make the job of cutting easier?
Scissor blades are wedges,
while the handles act as
levers. Moving the handles
brings together the blades,
generating a force that
makes the work of cutting
easier.
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Here are some other everyday compound machines:
• Axe:
• Combines a wedge (the axe head) with a lever (the handle).
• Swinging the lever transfers a powerful force to the wedge, which cuts more deeply
into an object than if you were merely to press the wedge against the object.
• Stapler:
• Combines a wedge (the staples) with a lever (the stapler top).
• Pressing down on the lever drives the wedges into a stack of paper, holding together
the individual papers.
• Hand-operated can opener:
• Combines a wedge (the blade) with a lever (the handle) attached to a wheel and axle.
• Rotating the lever transfers the force to the wheel and axle, which transfers the force
to the wedge, which cuts into and around the lid of the can.
• Wheelbarrow:
• Combines levers (the handles) with an inclined plane (the bucket) and a wheel
and axle.
• Lifting the levers transfers the force to the inclined plane and the wheel and axle,
allowing us to push a load more easily than if we were to carry it.
You probably also have examples of simple machines such as screws (screw), hammers
(lever), and pulls for opening and closing curtains (pulleys). For each simple or compound
machine, ask your child how each machine makes doing work easier.
Here are some questions to discuss with your child:
• What kind of simple machine is this? How does it make work easier?
• Is this actually a compound machine? If so, what simple machines is it made of?
• Do these machines cause you to do less work? Explain your answer.
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