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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.
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.
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.
(a) Which rider, Paul or Kathleen, is traveling faster at the
bottom?
(b) Which rider makes it to the bottom first?
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 weren’t there?
Machines
A machine is a device for multiplying
forces or simply changing the direction of
forces.
The Lever
Work Input = Work Output
Input Force x Input Distance = Output Force x Output Distance
A lever can be a force multiplier.
But no machine can multiply
work!!!!
Another Simple Machine:
Inclined Plane
If we gain nothing for work, why bother?
Pulley: the lever in “disguise”
Inversed Example: Bicycle
Efficiency
Efficiency = (useful energy output)/(total energy input)
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?
Comparison of Kinetic Energy
and Momentum
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?
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.
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.
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?
1. When you get your power bill, you are charged for the
number of kilowatt-hours that you have used. Is kilowatthour 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?