Questions

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1. In tryouts of the national bobsled team, each
   competing team pushes a sled along a level,
   smooth surface for 5 meters. One team brings a
   sled that is much lighter than the others.
   Assuming that this team pushes with the same
   force as the others, compare the kinetic energy
   of the light sled to that of the others after 5
   meters . Compare the momentum of the light sled
   to that of the others after 5 meters.
2. Suppose the rules were changed in previous
   question so that the teams pushed for a fixed
   time of 5 seconds rather that a fixed distance of
   5 meters. Compare the momentum of the light sled
   to that of the others after 5 seconds. Compare
   the kinetic energy of the sled to that of the
   others after 5 seconds.

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Questions

1. You have been asked to analyze a collision at a
   traffic intersection. Will you be better off to
   begin your analysis using conservation of
   momentum or conservation of kinetic energy? Why?
2. If a system has zero kinetic energy, does it
   necessarily have zero momentum? Give an example
   to illustrate your answer.
3. An object has a velocity toward the south. If a
   force is directed toward the north, will the
   kinetic energy of the object initially increase,
   decrease, or stay the same? Explain.

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Conceptual Example Speeds on Two Water Slides

• Two water slides at a pool are shaped
  differently, but have the same length and start
  at the same height h. Two riders, Paul and
  Kathleen, start from rest at the same time on
  different slides.
• Which rider, Paul or Kathleen, is traveling
  faster at the bottom?
• Which rider makes it to the bottom first?

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1. Does a car consume more fuel when its air
   conditioner is turned on? When its lights are on?
   When its radio is on while it is sitting on the
   parking lot?
2. Rows of wind-powered generators are used in
   various windy locations to generate electric
   power. Does the power generated affect the speed
   of the wind? That is, would locations behind the
   windmills have more wind if the windmills
   werent there?

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Machines
A machine is a device for multiplying forces or
simply changing the direction of forces.
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The Lever
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Work Input Work Output Input Force x Input
Distance Output Force x Output Distance
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A lever can be a force multiplier. But no
machine can multiply work!!!!
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Another Simple MachineInclined Plane
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If we gain nothing for work, why bother?
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Pulley the lever in disguise
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Inversed Example Bicycle
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Efficiency
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Efficiency (useful energy output)/(total energy
input)
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Question
Consider an imaginary miracle car that has a 100
efficient engine and burns fuel that has an
energy content of 40 mega joules per liter. If
the air drag and overall frictional forces on the
car traveling at highway speed are 500 N, what is
the upper limit in distance per liter the car
could go at this speed?
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Comparison of Kinetic Energy and Momentum
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Question
Suppose a hunter is confronted with a charging
bear. Which be more effective in knocking the
bear down a rubber bullet or a lead bullet of
the same momentum?
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Summary of Terms
Work. The product of the force and the distance
through which the body acted on by the force
moves. Power. The time rate of work. Energy. The
property of a system that enables it to do
work. Potential Energy. The energy that a body
possesses because of its position. Kinetic
Energy. Energy of motion. Work-Energy Theorem.
The work done on an object equals the change in
kinetic energy of the work.
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Conservation of Energy. Energy cannot be created
or destroyed it may be transformed from one form
into another, but the total amount of energy
never changes. Machine. A device that increases (
or decreases) a force or simply changes the
direction of a force. Conservation of Energy for
a Machine. The work output of any machine cannot
exceed the work input.
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1. Mountain highways often have emergency ramps for
   truckers whose brakes fail. Why are these covered
   with soft dirt or sand rather that paved?
2. Why can we not associate a potential energy with
   the frictional force as we did with the
   gravitational force?
3. A physics textbook is launched up a rough inline
   with a kinetic energy of 200 J. When the book
   comes momentarily to rest near the top of
   incline, it has gained 180J of gravitational
   potential energy. How much kinetic energy will it
   have when it returns to the launch point?

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1. When you get your power bill, you are charged for
   the number of kilowatt-hours that you have used.
   Is kilowatt-hour a unit of power or a unit of
   energy?
2. Athletes will sometimes run along the beach to
   increase the effect of their workouts. Why is
   running on soft sand so tiring?
3. Which of the following is not a unit of energy
   joule, Newton-meter, kilowatt-hour, watt?