Work and power and conservation

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Work and power and conservation

• Chapter 7
• Work and Kinetic Energy

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Work and Energy

• Work A quantity that measures the effects of a
  force acting over a distance.
• Work Force times distance
• WFD
• When a force is exerted on a body that moves the
  body from one position to another, work is done.

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For work to happenForce in the direction of
displacement

• Application of a force
• Movement of something by the force
• Measured in Joules
• Often in Mega joules or MJ

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Figure 7-1Work Force in the Direction of Motion
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72 kg patient 15 kg gurney, .6m/ss d2.5 How
much work?
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Figure 7-3Force at an Angle to Direction of
Motion Another Look
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Slip Sliding Away 75 kg person D 5m 2.5 M
heightHow much work is done?
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Hard at work

• Captain Peter
• at work in
• The old days
• Who is doing
• work

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Mechanical Energy

• May be in the form of potential energy or kinetic
  energy.
• May be the sum of the two.
• A pendulum converts potential gravitational
  energy to kinetic and back

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Chemical potential energy being released

• What forces
• are acting
• on the shuttle.

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Different types of energyHeat

• Fuel plus oxygen
• Exothermic heat given
• Stored chemical energy
• To light, and heat

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Energy changes state

• Nuclear
• to heat
• to wind
• to electrical

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Energy, Work in Joules

• Energy Work is when an object is moved, energy
  can be stored. Both are in Joules.
• Different types of energy kinetic, potential
• Potential chemical,
• Nuclear energy
• Electrical energy
• Magnetic energy
• Electromagnetic waves

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Work in Joules

• Force is
• Fma
• Units of newtons
• Joules
• Newtons times
• meters

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S I Units for work

• Units are Newtons meter or one Joule
• Energy is measured in Joules
• Different types of
• energy
• Conservation of
• energy

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Conceptual Checkpoint 7-1aWhich way is more work?
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Conceptual Checkpoint 7-1bWhich way is more work?
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Total work is the sum of the forces times the
distance on each axis

• We can add the forces in the x direction times
  the distance in the x direction to find the work
  done in the x direction

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Total work

• Total work is the sum of all of the work done in
  each direction
• Best method calculate the vector sum of all of
  the forces and then multiply times the
  displacement

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Total work
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Kinetic energy

• Kinetic energy is the energy of a moving object
  due to its motion

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An inherently dangerous ride

• Calculate the kinetic
• energy of the space
• Shuttle
• Typical low earth
• orbit re-entry speeds
• are near 17,500 mph

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• Calculate the kinetic energy
• The mass is 2,200,000 Kg
• Miles Per Hour (mph)Meters Per Second (mps)mph x
  0.447
• 7,822.5 m/s

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Total work done on an object is equal to the
change in its kinetic energy
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Table 7-2Typical Kinetic Energies
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Table 7-1Typical Values of Work
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Hit the Books Applied force is 60 N y 1.6 m,
4.1 kg, Final speed Ek and Ep
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Example 7-6Pulling a Sled 11N at 29 degrees,
6.4 kg sled find the work done and the final
speed after 2 m Assume starts with an initial
speed of .5m/s and no friction
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Conceptual Checkpoint 7-2Which takes more work?
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Potential energy

• Potential energy , generally due to gravity. The
  stored energy resulting from the relative
  positions of objects in a system.

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Gravitational potential energy

• Gravitational potential energy energy an object
  has due to its position above the surface of the
  earth.

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Gravitational potential energy

• Gravitational potential energy is
• the mass
• times the gravitational acceleration
• times the height.

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Gravitational potential energyequals the mass
times the acceleration (10 m/s2times the height
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Figure 7-7Work Done by a Non-Constant Force
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Figure 7-8aWork Done by a Continuously Varying
Force
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Figure 7-8bWork Done by a Continuously Varying
Force
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Figure 7-8cWork Done by a Continuously Varying
Forcecalculus
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Figure 7-10Work Needed to Stretch a Spring a
Distance x
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Figure 7-9Stretching a Spring
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Work to stretch or compress a spring.
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Stretching the Slinky Dog Work to stretch 1
meter is 2 J what is the KHow much work to
stretch to 2 meters
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Example 7-7bStretching the Slinky Dog
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Figure 7-11Work Done in Stretching a Spring
Average Force
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Figure 7-12The Work Done by a Spring Can Be
Positive or Negative
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Figure 7-12The Work Done by a Spring Can Be
Positive or Negative
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Power

• Power a quantity that measures the rate at which
  work is done
• Power equals work divided by time
• PW/T
• Units are Watts which are Joules per second

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Who is watt

• A unit of power called the Watt was named after
  James Watt. the Watt symbol is W, and it is equal
  to 1/746 of a horsepower, or one Volt times one
  Amp.

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• Watts company had a virtual monopoly over the
  production of steam-engines. Watt charged his
  customers a premium for using his steam engines.
  To justify this he compared his machine to a
  horse. 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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Watts
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Kw to Hp

• One horsepower hp is equal to one.75 kilowatts KW

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Different watts

• Todays 100 watts
• Three way
• 50, 100, 150

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Edisons invention

• Labeled
• 100 watts

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POWER!!!!!!!!!!!!

• Power is defined as the ability to do work over a
  period of time.
• Pw/t
• Units joules/second, or watt

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Table 7-3Typical Values of Power
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Passing Fancy mass of your car 1,300 Kg time 3
seconds, acceerate frp, 13.4 to 17.9
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Figure 7-13Driving Up a Hill
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Figure 7-14Problems 7-12 and 7-62