Download Work and Power

Work, Power, and
Machines
Last night, you read a chapter for
your language arts homework.
Did you do work??
Not really!
 According to science, you only do work when:
 A force is applied to an object
 That object moves in the same direction as the force applied
 Note: You did do work when you turned the pages in the book,
but your overall process of reading was not work.
What is Work?
 When you apply a force and the object moves in the
same direction as the force, you do work.
 Ex: You push a cart and it moves forward.
 Reading a book is not doing work because you are not
applying a force on the book or making it move.
 When you transfer energy to something, you do work.
 Ex: Someone bowling is doing work because they have
applied a push force on the bowling ball which makes it
move forward. The bowler transferred energy to the ball,
causing it to start moving.
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Don’t get tricked!
 In order to do work on an object, it has to move in the
same direction as the force you apply…
 When you pick up books off the top of a desk, you do
work:
 Force applied? YES
 Object moves? YES
 Direction of force: UP
 Direction of motion: UP
Don’t get tricked!
 In order to do work on an object, it has to move in the
same direction as the force you apply…
 When you walk through the hallway and carry your
books, you do not do any work on the books:
 Force applied? YES
 Object moves? YES (with you)
 Direction of force: UP
 Direction of motion: FORWARD
 The direction of the force and distance traveled are not the
same, so no work is done.
Is It Work?
Situation
Carrying a bag of groceries
Picking up a bag of groceries
Pushing in your chair
Sitting in your chair
Pulling a wagon
Pushing against a brick wall
Work? (Yes or No)
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Try some on your own…
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Same Work, Different Forces
 Rock climb straight up the mountain
 Shorter distance
 Greater force required to pull yourself straight up
 Hike along winding slope to the top
 Greater distance
 Less force required to make it to the top
 Same amount of work done!
Calculating Work
 Imagine you are lifting weights. You are able to lift
a186 Newton bar a distance of 2 meters. How much
work do you do?
 Remember that work occurs when an object moves in the
direction of the force applied, so…
 Work = Force x Distance
 Units: Newtons x meters
or
W=Fxd
so
N x m = Joules (J)
 Unit for work: Joules (pronounced like jewels)
Calculating Work
Imagine you are lifting weights. You are able to lift a186
Newton bar a distance of 2 meters. How much work do
you do?
CLUES
FORMULA
SUBSTITUTE
You do 372 J (joules) of work.
ANSWER
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Power!
 What is more powerful…you knocking down a house
with a hammer, or a bulldozer knocking down a house?
 The bulldozer, because it applies more force and does
more work in less time!
 Anything that does the same amount of work in less
time has more power!
 Anything that does more work in the same amount of
time has more power!
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What is Power?
 Power is defined as the rate at which energy is
transferred or work is done.
 In simple terms, this means the amount of time it takes
to do work.
 If you do the work in less time, you are more powerful
and have a high power output.
 If you do the work in more time, you are less powerful
and have a low power output.
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Calculating Power
 Because power is the amount of work you do in a
certain amount of time, it is calculated with the formula:
Power = work divided by time
P = W/t
 The unit of power is:
 Joules/second, represented as watts (w)
Calculating Power
 A light bulb is on for 12 seconds and uses 1,200 J of
electrical energy during that time. What is the wattage
(power) of the light bulb?
CLUES
FORMULA
SUBSTITUTE
ANSWER
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How can work get easier?
 We use machines to make work easier.
 A machine is a device that makes work easier. This is
because they change the size or direction of the force
you need to apply in order to do the work.
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cutting a steak with a sharp knife or tearing it by hand
bulldozer vs. breaking apart by hand
snow blower vs. shoveling
electric mixer vs. mixing cookie dough by hand
Types of Simple Machines
Lever
Inclined
Plane
Wedge
Screw
Pulley
Wheel and
axle
Description
A bar that
pivots at a
fixed point
A straight,
slanted
surface
Consists of
two inclined
planes
An inclined
plane
wrapped in
a spiral
around a
cylinder
A grooved
wheel that
holds a
rope or
chain
Consists of
two circular
objects of
different
sizes
Purpose
Raise an
object or
load
Raise a
heavy
object
Used for
cutting or
splitting
Used to
fasten
things
together
Used to lift
something
Used to
turn or
open/close
an object
Examples
Bottle
opener,
hammer
Ramp
Knife, ax
Screw
Flagpole
hoist,
open/close
blinds
Door knob,
steering
wheel,
wrench
Lever
Wheel and Axle
Pulley
Wedge
Inclined Plane
Screw
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Compound Machines
 Also known as complex machines
 Made of two or more simple machines
 Examples: bicycle, can opener, pencil sharpener, lawn
mower, wheel barrow
Rube Goldberg was an inventor
who created unique devices
made out of many different types
of simple machines.
How many machines can
you identify in the
following video?
Now, take a Work and Power
calculation sheet and
practice your math skills!
~Be sure to show your work using
the 4 steps.
~Be careful with your new units
and formulas!
~Complete both sides of the sheet.