Work, Energy and Power

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Work, Energy and Power
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Work and Energy

• Work and energy
• Energy gives us one more tool to use to analyze
  physical situations. When forces and
  accelerations are used, you usually freeze the
  action at a particular instant in time, draw a
  free-body diagram, set up force equations, figure
  out accelerations, etc. With energy the approach
  is usually a little different. Often you can look
  at the starting conditions (initial speed and
  height, for instance) and the final conditions
  (final speed and height), and not have to worry
  about what happens in between.
• Work and energy
• Whenever a force is applied to an object, causing
  the object to move, work is done by the force. If
  a force is applied but the object doesn't move,
  no work is done if a force is applied and the
  object moves a distance d in a direction other
  than the direction of the force, less work is
  done than if the object moves a distance d in the
  direction of the applied force.
• The physics definition of "work" is
• The unit of work is the unit of energy, the joule
  (J). 1 J 1 N m.
• Work can be either positive or negative
• if the force has a component in the same
  direction as the displacement of the object, the
  force is doing positive work.
• If the force has a component in the direction
  opposite to the displacement, the force does
  negative work.
• If you pick a book off the floor and put it on a
  table, for example, you're doing positive work on
  the book, because you supplied an upward force
  and the book went up. If you pick the book up and
  place it gently back on the floor again, though,
  you're doing negative work, because the book is
  going down but you're exerting an upward force,
  acting against gravity.
• Example 1,2 page 149-150

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Work and Energy

• Kinetic energy
• An object has kinetic energy if it has mass and
  if it is moving. It is energy associated with a
  moving object, in other words. For an object
  traveling at a speed v and with a mass m, the
  kinetic energy is given by
• The work-energy principle
• There is a strong connection between work and
  energy, in a sense that when there is a net force
  doing work on an object, the object's kinetic
  energy will change by an amount equal to the work
  done
• Note that the work in this equation is the work
  done by the net force, rather than the work done
  by an individual force.
• Gravitational potential energy
• Let's say you're dropping a ball from a certain
  height, and you'd like to know how fast it's
  traveling the instant it hits the ground. You
  could apply the projectile motion equations, or
  you could think of the situation in terms of
  energy (actually, one of the projectile motion
  equations is really an energy equation in
  disguise).
• If you drop an object it falls down, picking up
  speed along the way. This means there must be a
  net force on the object, doing work. This force
  is the force of gravity, with a magnitude equal
  to mg, the weight of the object. The work done by
  the force of gravity is the force multiplied by
  the distance, so if the object drops a distance
  h, gravity does work on the object equal to the
  force multiplied by the height lost, which is
• work done by gravity W mgh (h height lost
  by the object)
• An alternate way of looking at this is to call
  this the gravitational potential energy. An
  object with potential energy has the potential to
  do work. In the case of gravitational potential
  energy, the object has the potential to do work
  because of where it is, at a certain height above
  the ground, or at least above something.
• Example 1 page 149

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Work and Energy
5
Work and Energy

• Conservation of mechanical energy
• We'll take all of the different kinds of energy
  we know about, and even all the other ones we
  don't, and relate them through one of the
  fundamental laws of the universe.
• The law of conservation of energy states that
  energy can not be created or destroyed, it can
  merely be changed from one form of energy to
  another.
• Energy often ends up as heat, which is thermal
  energy (kinetic energy, really) of atoms and
  molecules.
• Kinetic friction, for example, generally turns
  energy into heat, and although we associate
  kinetic friction with energy loss, it really is
  just a way of transforming kinetic energy into
  thermal energy.
• The law of conservation of energy applies always,
  everywhere, in any situation. There is another
  conservation idea associated with energy which
  does not apply as generally, and is therefore
  called a principle rather than a law. This is the
  principle of the conservation of mechanical
  energy.
• Example 10 page 163

If my PE isnt converted to KE, Ill be up here
for a long time
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Work and Energy
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Work and Energy

• Conservation of energy simulation
• Circular Mass
• Attach to curved slot joint
• Make it look like a roller coaster
• Measure
• Velocity
• Acceleration
• Kinetic energy
• Gravitational acceleration
• Print out results
• Note starting potential energy and final kinetic
  energy (KE) potential energy (PE)

8
POWER

• Power
• Being able to do work is not just what's
  important how fast you can do work is also an
  important factor. Power is the measure of how
  fast work is done. Computers have more
  calculating power than we do a sports car
  generally has a more powerful engine than an
  economy car. Power is the rate at which work is
  done and the rate at which energy is used. The
  unit for power is the watt (W).
• An interesting calculation is the average power
  output of a human being. This can be determined
  from the amount of energy we consume in a day in
  the way of food. Most of us take in something
  like 2500 "calories" in a day, although what we
  call calories is really a kilocalorie assuming
  we use up all this energy in a day (a reasonable
  assumption considering we'll have to eat
  tomorrow, too) we can use this as our energy
  output per day.
• First, take the 2.5X106 cal and convert to
  Joules, using the conversion factor 4.18 J / cal.
  This gives roughly 1 x 107 J. Figuring out our
  average power output, we simply divide the energy
  by the number of seconds in a day, 86400, which
  gives a bit more than 100 W. In other words, on
  the average, we're just a little brighter than
  your average light bulb.
• Eat a good breakfast!
• Work example 13 page 166