WORK

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WORK AND POWER
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WORK
TRANSFER OF ENERGY
When a force causes a displacement
W F d
CLIP
Unit Joule
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James Prescott Joule (December 24, 1818October
11, 1889) was an English physicist.
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• Educated at home
• sent to Cambridge at the age of 16to study with
  the British chemist John Dalton

• 1850-established the equivalence between amounts
  of heat and mechanical work with his famous
  "paddle-wheel experiment".
• Also established the cooling effect that occurs
  when a gas is rapidly expanded, an effect that is
  used in refrigeration systems.

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• Shared in discovering the law of the conservation
  of energy.

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• 1840- stated a law, now called Joule's Law, that
  heat is produced in an electrical conductor.
• The International unit of energy, the joule, is
  named in his honor.

Among his many inventions are "arc" or electrical
welding and the displacement pump.
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WORK
For there to be work on an object, there must be
a displacement and the force must cause that
displacement
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Is work done here?
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• pushing on a wall
• pushing a boat out to sea
• lifting up a box
• holding up a box

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Calculating Work
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When doing a chin-up, a physics student lifts her
40-kg body a distance of 0.25 meters in 2
seconds. How much work is done by the student's
biceps?
W F d (400 N) (0.25 m)
W 100 J
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Work has nothing to do with the amount of time
that this force acts to cause the displacement.
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Sometimes, the work is done very quickly and
other times the work is done rather slowly.
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POWER
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• The rate of energy transfer.
• How fast work is done.

• Machines with different power ratings do the same
  amount of work...just in different amounts times.

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Son of a merchant, born in Greenock, Scotland, in
1736
-1757established his own mathematical
instrument-making business
Watt discovered how he could make the steam
engine more efficient by cooling the used steam
in a condenser that was separate from the main
cylinder. This made them faster, safer, and more
economical, resulting in steam overtaking
water-power as the main source of energy.
James Watt
Unit of power is the Watt
Watt calculated that a horse exerted a pull of
180 lb., therefore, when he made a machine, he
described its power in relation to a horse, i.e.
"a 20 horse-power engine".
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watt (W)
1W 1 J/s
One watt is equal to a power rate of one joule of
work per second of time.
1Kilowatt 1000 Watts
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Calculating Power
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When doing a chin-up, a physics student lifts her
40-kg body a distance of 0.25 meters in 2
seconds. What is the power delivered by the
student's biceps?
The work done to lift her body is
W F d (400 N) (0.25 m)
W 100
N-m W 100 J The power is the
work/time ratio which is (100 J) / (2 seconds)
50 Watts.
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Torque
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• A measure of how much a force acting on an object
  causes that object to rotate.

• A torque is a force exerted at a distance from
  the axis of rotation the easiest way to think of
  torque is to consider a door. When you open a
  door, where do you push? If you exert a force at
  the hinge, the door will not move the easiest
  way to open a door is to exert a force on the
  side of the door opposite the hinge, and to push
  or pull with a force perpendicular to the door

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Torque
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