Transcript FORCE & MOTION WORK & POWER

WORK & POWER
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Work
Work has a special meaning in
science. It is the product of the
force applied to an object and
the distance the object moves.
The unit of work is the joule (J).
Work = force x distance
W=Fxd
force = newtons
distance = meters
W
F
d
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Work
Work is done on an object when
something exerts a force on the
object that causes it to move some
distance in the direction of the force.
-Work is not done if there is no
motion.
-Work is not done if the motion is not
in the same direction as the force.
Power is the amount of work
done per unit of time. The unit
for power, joules/second, is the
watt.
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Power
Power = work/time
W
P t
work = joules
time = seconds
Description
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Machines and Work
 Machines make work easier but do not change the
amount of work that is done. Machines either change
the amount of force you exert, the distance over
which you exert your force, or change the direction
in which you exert your force.
 Multiplying force: Less force is required but it must
be applied over a longer distance.
 Multiplying distance: Machine allows you to work
over a shorter distance but more force is required
 Changing direction: This only changes direction not
the force required or distance applied.
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Machines
An instrument that makes work
easier is called a machine.
Machines are not limited to the
complicated devices you may be
thinking of – car engines,
airplanes, and computers.
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Simple Machines
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Lever
 Try pulling a really stubborn weed out of the ground. You
know, a deep, persistent weed that seems to have taken
over your flowerbed. Using just your bare hands, it might
be difficult or even painful. With a tool, like a hand shovel,
however, you should win the battle. Any tool that pries
something loose is a lever. A lever is an arm that "pivots"
(or turns) against a "fulcrum" (or point). Think of the claw
end of a hammer that you use to pry nails loose. It's a
lever. It's a curved arm that rests against a point on a
surface. As you rotate the curved arm, it pries the nail
loose from the surface. And that's hard work!
 Explanation and a Simple explanation
 Powerpoint from Ohio State
 3 types of levers and more
 Interesting but I wouldn’t try it
 Fun explanation and experiments
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Experiment
Supplies: ruler, pencil, 4 pennies

Put the pencil under the middle of the ruler so the ruler
balances. The pencil is the fulcrum of the lever. Put one penny
at one end of the ruler. The end with the penny is called the
resistance arm because it has the weight that you are trying to
lift. What happens to the lever and why?

Where do you need to put the second penny to make the ruler
balance? Try.

The end of the lever which you push down to do the lifting is
called the effort arm. Why

Place a second penny on top of the first one on the resistance
arm of the lever. The resistance is now two pennies. How many
are on the effort arm? Is the lever balanced now? Explain.

Without adding any more pennies make the lever balance by
sliding the fulcrum toward one end of the ruler. Which way did
it move to balance the ruler?

Add a third penny to the resistance arm so the lever is out of
balance again. Which way would you have to move the fulcrum to
balance the lever? Try. Which arm is longer – the resistance
arm or the effort arm?
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Archimedes
 While Archimedes did not invent the lever,
he gave the first rigorous explanation of
the principles involved. Those principles are:
the transmission of force through a fulcrum
and moving the effort applied through a
greater distance than the object to be
moved. His Law of the Lever states:
Magnitudes are in equilibrium at distances
reciprocally proportional to their weights.
The results of his research and work on
levers caused him to remark: "Give me a
place to stand on, and I will move the
Earth."
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Inclined Plane
 A plane is a flat surface. For example, a
smooth board is a plane. Now, if the plane is
lying flat on the ground, it isn't likely to
help you do work. However, when that plane
is inclined, or slanted, it can help you move
objects across distances. And, that's work!
A common inclined plane is a ramp. Lifting a
heavy box onto a loading dock is much easier
if you slide the box up a ramp--a simple
machine.
 Explanation
 Using Legos
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Wedge
Instead of using the smooth side of
the inclined plane, you can also use
the pointed edges to do other kinds
of work. For example, you can use the
edge to push things apart. Then, the
inclined plane is a wedge. So, a wedge
is actually a kind of inclined plane. An
axeblade is a wedge. Think of the
edge of the blade. It's the edge of a
smooth slanted surface. That's a
wedge!
Explanation
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Screw
 Now, take an inclined plane and wrap it
around a cylinder. Its sharp edge becomes
another simple tool: the screw. Put a metal
screw beside a ramp and it's kind of hard to
see the similarities, but the screw is
actually just another kind of inclined plane.
How does the screw help you do work?
Every turn of a metal screw helps you move
a piece of metal through a wooden space.
And, that's how we build things!
 Explanation
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Wheel and Axel
 The rotation of the lever against a point pries
objects loose. That rotation motion can also do other
kinds of work. Another kind of lever, the wheel and
axle, moves objects across distances. The wheel, the
round end, turns the axle, the cylindrical post,
causing movement. On a wagon, for example, the
bucket rests on top of the axle. As the wheel
rotates the axle, the wagon moves. Now, place your
pet dog in the bucket, and you can easily move him
around the yard. On a truck, for example, the cargo
hold rests on top of several axles. As the wheels
rotate the axles, the truck moves.
 Explanation
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Pulley
 Instead of an axle, the wheel could also rotate a rope or
cord. This variation of the wheel and axle is the pulley. In a
pulley, a cord wraps around a wheel. As the wheel rotates,
the cord moves in either direction. Now, attach a hook to
the cord, and you can use the wheel's rotation to raise and
lower objects. On a flagpole, for example, a rope is
attached to a pulley. On the rope, there are usually two
hooks. The cord rotates around the pulley and lowers the
hooks where you can attach the flag. Then, rotate the cord
and the flag raises high on the pole.
 Explanation
 Detailed information
 Pictures
 Leonardo da Vinci
 Diagrams
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Pulley Experiment
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Simple machines
Practice
Lots of information
Simple explanation (try sketching activity)
Review (try the activities and picture review)
Univ of Utah review
Rube Goldberg
Leonardo’s Mysterious Machines
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Mechanical Advantage
The mechanical advantage is the
number of times a machine
multiplies your effort force or
the ratio of force output to
force input
Calculating
Practice problems
Finding MA
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Calculating MA
 The mechanical advantage of a machine is
the number of times the force exerted is
multiplied by the machine.
 Mechanical advantage = Output force

Input force
 -If the mechanical advantage is greater
than 1 the machine multiplies force.
 -If the mechanical advantage is less than 1
the machine multiplies distance.
Ideal Mechanical
Advantage
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•
•
Mechanical Advantage without
FRICTION!
MA = Input distance (m)
–
Output distance (m)
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Energy
Energy can be defined as the
ability to cause change or do
work. Without energy no work
could be done.
Simple explanation
Roller coaster example
Energy & Work
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 The seven forms of energy are:
 Radiant – kinetic ex: sunlight, x-rays, microwaves
(x-ray site, microwave website)
 Electrical – kinetic ex: generator, lightning
 mechanical – kinetic ex: windmill, gas motor
(mechanical & chemical) website
 thermal – kinetic ex: fire burning, hot plate
 chemical – potential ex: gasoline, food
 sound – kinetic ex: vibration
 Nuclear – potential ex: radioactive substances
Forms of Energy
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 Identify the form of energy used in each picture.
Look at the previous slide if you’re not sure.
Forms of Energy
Are You Right?
mechanical
sound
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 Identify the form of energy used in each picture.
Look at the previous slide if you’re not sure.
mechanical
electrical
radiant
electrical
mechanical
radiant
mechanical
sound
radiant
chemical
mechanical
thermal
radiant
thermal
nuclear
radiant
radiant
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Potential energy
Potential energy is energy that
is stored in an object. If you
stretch a rubber band, you will
give it potential energy. As the
rubber band is released,
potential energy is changed to
motion.
Potential energy
Boulder sitting at the top of a hill
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This is stored energy due to
position.
energy = joules
weight = newtons
height = meters
Potential Energy = weight x height
PE = w x h
website
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Kinetic Energy
 Kinetic energy is energy of motion. A rubber band flying
through the air has kinetic energy. When you are walking
or running your body is exhibiting kinetic energy. Potential
energy is converted into kinetic energy. Before the yo-yo
begins its fall it has stored energy due to its position. At
the top it has its maximum potential energy. As it starts to
fall the potential energy begins to be changed into kinetic
energy. At the bottom its potential energy has been
converted into kinetic energy so that it now has its
maximum kinetic energy. A waterfall has both potential and
kinetic energy. The water at the top of the waterfall has
stored potential energy. When the water begins to fall, its
potential energy is changed into kinetic energy. This
change in energy happens at Niagara Falls where it is used
to provide electricity from the transformation of
mechanical and electromagnetic energy to parts of the
northeastern United States.
 Another explanation , Brain pop movie
Kinetic energy
Boulder is rolling
Kinetic energy = ½ mass x velocity2
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This is the energy of motion, it
depends on mass and velocity.
KE = ½ m * v2
energy = joules
mass = kilograms
velocity = meters/second
website
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Law of Conservation
of Mass & Energy
For a closed system, the sum of
the potential energy and the
kinetic energy is a constant. As
the potential energy decreases,
the kinetic energy increases.
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Energy conversions
Energy doesn’t disappear, it
changes from potential to kinetic
energy.
Some energy may leak out of the
system into the surrounding
environment.
Explanation
Longer explanation
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Experiment
Janice VanCleave’s 201 Awesome, Magical, Bizarre and Incredible Experiments
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Terrible Weekend

After much arm-twisting, Alex and Andrew have finally been given
permission by their parents to go on a weekend camping trip. They load up
their gear and head out of town on foot. After walking for nearly two hours,
they finally reach their destination; a secluded spot in a small wooded area
in the country. After a quick rest they set up their camp, consisting of a
small tent and a pile of firewood. Unfortunately their matches have gotten
a bit wet and they are unable to get a campfire started. That's OK...They
are hungry enough to eat cold hot dogs. Darkness is soon upon them and
they begin feeling a bit isolated without the warm comfort of a campfire.
They decide to use their flashlight to brighten up the tent and have a quick
card game. But after only 10 minutes of use, the light becomes dimmer and
finally goes out. What bad luck. They forgot the extra batteries! Things go
from bad to worse as the sound of distant thunder rolls toward them. Soon
the sky is lit with bright flashes of lightening and the ground seems to
shake with the thunder. Not a good night to be out. Andrew wishes he were
back home, but Alex insists they are having a good time. As the rain
continues to pour down, water begins dripping through the roof of the tent.
Alex sees this as a bad sign. The lightning is so intense that it seems to be
steady. "Wait a minute", says Andrew, "that is a steady light". "Someone is
coming, where can we hide?" But before an escape can be planned, the tent
unzips and in steps Dad. Alex feels a rush of relief. Although he would never
admit it, he was very happy to be "rescued" on a night like this!

Can you decide when energy is being converted form one form to another in
this story?
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Mechanical Efficiency
Efficiency compares the output
work to the input work and is
expressed as a percent.
Machines can lose efficiency to
frictional forces. Efficiency is
never greater than 100%.
Efficiency = Output work X 100%
Input work
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Review
Practice problems
More practice problems
Physics book, explanation and
problems
Amusement park physics
Seatbelt physics
Work-energy
Review