Work

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Work
No Work

• Work Force x Distance
• The force must be in the direction
• of the motion, or no work is done.

Work
W
F
D
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The unit of work is the Joule

• Force x Distance Work
• 1 Newton x 1 meter 1 Newton meter
• 1 Newton meter 1 Joule

James Prescott Joule
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Example 1

• A high jumper weighs 700 newtons. What work does
  the jumper perform in jumping over a bar 2.0
  meters high?
• Answer W F x d
• W 700N x 2.0 m
• 1400 nm
• 1400 Joules

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No work is done if there is no distance!

• The statue of liberty has been holding up her
  torch for an awfully long time. How much work
  has she done?
• Answer 0! Although it takes a force to hold the
  torch against the force of gravity, there is no
  motion so no work is done.

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Which does more work?
Frictionless
The net work is the force (weight) of the cart X
the vertical distance. This is the same in all
three cases. In the first, the force is less,
but distance is greater to reach the same
vertical height.
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Example 2

• A force of 200N is required to push a lawn mower.
  If 4000 J of work is performed on the lawnmower,
  how far does it move?
• Answer
• 4000Nm 200N D
• 20m

W
F
D
4000J
D
200N
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Remember

• For work to happen the force has to be applied in
  the SAME direction as the motion.

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Power
W

• Work done per unit of time
• Power Work
• Time
• Power Force x distance
• Time

P
T
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Power

• An object that has more power than another object
  does more work in the same time.
• You need more power to
• run up the stairs than to walk
• because it takes you less time
• to do the same work.

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The SI unit of power is the WATT (W)

• Power Work
• Time
• 1 watt 1 Joule
• 1 Second

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A Watt is small!

• Usually measured in kilowatts(kW) 1,000W
• Another unit of power
• Horsepower (not an SI unit)
• 1horsepower 746 watts

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Example 3

• A crane lifts a car into a junk pile in 10
  seconds. What is the cranes power if 120,000 J
  of work are performed?
• Answer Power Work
• Time
• 120,000J
• 10 sec
• 
  12,000 Watts

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Example 4

• A 750 N diver does a somersault off a 10m
  platform. It takes her 1.5 seconds to hit the
  water. What is her power?
• Since work is Force x distance, the power formula
  can be written as
• Power Force x distance
• Time

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Answer

• Power Force x distance
• Time
• Power 750N x 10m
• 1.5 sec
• Power 7500N m
• 1.5 sec
• 5000 Watts

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Other units of power

• 1 kilowatt 1000 watts
• 1 horsepower 746 watts
• In the previous example, how many kilowatts power
  was generated? How much horsepower?

• Answer
• 5000 watts 1000 kw/w 5 kilowatts
• 5000 watts 746 hp/w 6.7 horsepower

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What is a Machine?

• A device that allows you to do work in a way that
  is easier.
• It changes at least one of 3 factors
• The amount of force you exert
• The distance over which you exert your force
• The direction in which you exert your force.

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Input Output Force

• Input force- The force you exert on the machine.
• The input force moves the machine a certain
  distance, called the input distance.
• Output force- The force the machine exerts on an
  object.
• The machine does work by exerting a force over
  another distance, called the output distance.

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Input Output Work

• Input work input force x input distance
• Output work Output force x Output
  distance
• The amount of output work can never be greater
  than the amount of input work.

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Changing Force

• In some machines, the output force is greater
  than the input force
• This is possible by increasing the distance.
• What kind of machine allows you to exert a
  smaller input force?
• Ramp, faucet knob.

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Changing Distance

• In some machines, the output force is less than
  the input force
• Why would you use a machine like this?
• Hockey stick, chopsticks, riding a bike on a high
  gear.

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Changing Direction

• Some machines dont change either the force of
  the distance.
• What is the advantage of using a machine like
  this?
• It is easier to pull down then to push up.

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(No Transcript)
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Mechanical Advantage

• A machine's mechanical advantage is the number of
  times a machine increases a force.
• Formula
• mechanical advantage (MA) Output Force
• Input Force
• There is no unit for MA.

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Mechanical Advantage

• (IMA) Ideal MA This is the MA of a machine in a
  world with no friction, and no force is lost
  anywhere.
• (AMA) Actual MA This is simply the MA of a
  machine in the world as we know it.
• - Force is lost due to
  friction.
• - It is transferred into
  thermal energy.
• Can we have an ideal machine?

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Increasing Force

• When the output force is greater than the input
  force, the MA is greater than 1.
• Suppose you exerted a force of 10 N on a
  hand-held can opener and the can-opener exerts a
  force of 30 N on the can. What is the MA of the
  can opener?
• MA Output force
• Input Force
• MA 30N
• 10N
• MA 3

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Increasing Distance

• For a machine to increase distance, the output
  force is less than the input force. The MA is
  less than 1
• Suppose you applied a force of 20N and the
  machine applied a force of 10 N. What is the MA?
• MA Output force
• Input Force
• MA 10N
• 20N
• MA 0.5
• The MA is less but the machine would exert the
  force over a longer distance.

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Changing Direction

• What can you predict about the MA of a machine
  that changes the direction of the force?
• If ONLY the direction is change the input force
  will be the same as the output force so the MA
  will always be 1

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Efficiency

• In real world situations, the output work is
  always less than the input work because some work
  is wasted overcoming friction.
• Efficiency compares the output work to the input
  work.
• Efficiency Output work x100
• Input work
• Efficiency is expressed as a percent

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Try it out

• You do 20 J of work while using a hammer. The
  hammer does 18J of work on the nail. What is the
  efficiency of the hammer?
• Efficiency Output work x100
• Input work
• Efficiency 18J x100
• 20J
• Efficiency .9 x100 90

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• You do 250,000J of work to cut a lawn with a hand
  mower. If the work done by the mower is 200,000J,
  what is the efficiency of the lawn mower?
• Efficiency Output work x100
• Input work
• Efficiency 200,000J x100
• 250,000J
• Efficiency .8 x100 80

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Is there a machine with 100 efficiency?

• Unfortunately, there is no such machine. In all
  machines some work is transferred into thermal
  energy due to friction.
• Ideal vs. Actual
• All machines have an efficiency of less than 100