Work, Power

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What do you think of when you hear the words

• Work, Power Energy?

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Work Force x Distance
W Fd The distance is the displacement. Only the
force parallel to the displacement does work
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Work

• Force MUST be applied in the direction of motion
• The unit for work is the Newton-meter which is
  also called a Joule.
• 1 joule is equal to a force of 1 N exerted over a
  distance of 1 m

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Work or No Work
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m 10kg
In this case, the weight does positive work
d 2 m
Work mgd (100N)(2m) Work 200 Nm 200J
m 10kg
mg 100N
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FA
m 10kg
In this case, the weight does negative work
mg 100N
d 2 m
Fa
Work -mgd -(100N)(2m) Work -200 Nm -200J
m 10kg
mg 100N
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Power

• measured in watts (W)
• One watt of power is used when one joule of work
  is done in one second.

http//www.cartoonstock.com/newscartoons/cartoonis
ts/pto/lowres/pton114l.jpg
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Power

• Power is equal to the amount of work done per
  unit time.

• The unit for power is the Joule/second which is
  also called a Watt.

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What is horsepower?

• 1 horsepower 746 Watts
• A horse exerting 1 horsepower can raise 330
  pounds of coal 100 feet in a minute, or 33 pounds
  of coal 1,000 feet in one minute, or 1,000 pounds
  33 feet in one minute.

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Energy

• Universe is made up of matter and energy.
• Energy is the mover of matter.
• Energy has several forms, some examples
• Mechanical
• Kinetic
• Potential
• Electrical
• Heat
• Chemical

http//tiki.oneworld.net/energy/energy.html
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Mechanical Energy

• Energy contained by an object due to its motion
  or its stored energy of position.
• It can either be potential energy or kinetic
  energy.
• All forms of energy are measured in joules (J).
• ME KE PE

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Conservation of Energy

• Energy cannot be created nor destroyed
• Energy may be transformed from one form into
  another, but the total amount of energy never
  changes.
• So why worry about conserving energy?

http//sol.sci.uop.edu/jfalward/physics17/chapter
3/chapter3.html
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Conservation of Mechanical Energy

• KE (before) PE (before) KE (after) PE
  (after)
• Why does a dropped ball not return to its
  original elevation?

http//www.science-projects.com/Drop/DropBalls.htm
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Energy Transformation of a Pendulum
http//www.glenbrook.k12.il.us/gbssci/Phys/mmedia/
energy/pe.html
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Potential Energy

• (PE) stored chemical energy or energy of
  position.
• An objects ability (potential) to do work by
  virtue of its position.
• Types elastic, gravitational and chemical.
• Examples
• Rubber bands
• Springs
• Bows
• Batteries
• Gravity?

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Gravitational Potential Energy

• PE Weight height Fwh mgh
• PE m g h
• Question
• How much potential energy does a 10kg mass have
  relative to the ground if it is 5 meter above the
  ground?

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Potential Energy

• The same work is done on each block.  What
  mattersis the final elevation, not thepath
  followed

http//sol.sci.uop.edu/jfalward/physics17/chapter
3/chapter3.html
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Kinetic Energy

• The energy of motion.
• Kinetic Energy ½ mass velocity2

KE ½ mv²
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How much work does gravity do on the falling mass?
PE mgh
mg
h
h
Work mgh
mg
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How much energy does the mass have at the bottom
of its fall, just before it hits the ground?
PE mgh
mg
h
h
Kinetic energy
mg
v
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PE mgh
If potential energy at top of path is 100 J, how
much kinetic energy does the ball have just
before it hits the ground and comes to a stop?
mg
h
KE ½ mv2
mg
v
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Check for Understanding

• Determine the kinetic energy of a 1000-kg roller
  coaster car that is moving with a speed of 20.0
  m/s.

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Determine the Unknown Information
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More Energy Transfer

• High speed winds are used to do work on the
  blades of a turbine at the so-called wind farm.
• Mechinacal Energy from the air gives the air
  particles the ability to apply a force to the
  blades.
• As the blades spin, their energy is converted
  into electrical energy (a non-mechanical form of
  energy) and supplied to homes and industries in
  order to run electrical appliances.

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Energy Transfer...

• This diagram shows that the potential energy of
  the boy is changing as he swings.
• Explain what is happening, in other words how is
  the energy changing?

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(No Transcript)
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Mechanical Energy Conversion

• Explain the following diagram
• What is happening
• Explain the conversion of KE to PE to KE to PE
• http//www.funderstanding.com/coaster

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Work/Energy Relationship

• If you want to move something, you have to do
  work.
• The work done is equal (ideally) to the change in
  kinetic energy.
• what is ideal? is this actual?
• W DKE

http//sol.sci.uop.edu/jfalward/physics17/chapter
3/chapter3.html
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Work-Energy Relationship

• What happens to your stopping distance when you
• Double your speed?
• Triple your speed?

http//www.thegrid.co.uk/index_files/Mickel-crash-
brands.jpg
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Joule to Joule
http//www.glenbrook.k12.il.us/gbssci/Phys/mmedia/
energy/cs.html
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Truck Collision

• In a head-on collision
• Which truck will experience the greatest force?
• Which truck will experience the greatest change
  in velocity?
• Which truck will experience the greatest
  acceleration?
• Which truck would you rather be in during the
  collision? WHY?

http//hyperphysics.phy-astr.gsu.edu/hbase/truckc.
htmlc1
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• Imagine standing on your deskif you jumped off,
  how fast would you be moving just prior to
  reaching the floor?
• How much work would you do on the floor?
• What information do you need to know?
• G
• U
• E
• S
• S

http//school.discoveryeducation.com/clipart/image
s/in-desk.gif