Chapter 8 Conservation Laws Flashcards
Unchanging in time. A quantity is “conserved” if the amount of that quantity does not change in time, even though processes may be changing its form.
Conserved Quantity
The elementary particles of which protons and neutrons consist. A proton and a neutron each consist of three quarks.
Quark
The total number of protons + neutrons in the nucleus of an atom.
Atomic mass number
An object’s mass times its velocity. Measures the amount of motion in a straight line.
Linear Momentum
Angular momentum is a quantity that measures the amount of rotational motion an object has.
Angular Momentum.
In terms of momentum what does a force do when it acts on an object? How does the principle of conservation
of momentum apply in the collision between two objects? Answer specifically in the case of objects colliding
with different masses and different initial speeds.
When two objects interact, a force, acting for a certain time between them, produces an impulse which
changes the momentum of both objects. Whatever momentum one object gain, the other must have an
equal, but opposite change. It does not matter what their relative speeds are. The change in the
momentum of the two must be equal and opposite.
The principle of conservation of momentum only applies to objects that stay intact. If an object were to
dissociate or explode, then the total momentum of that object as measured from the momentum of the sum of
its parts would have had a net change in momentum owing to the explosion or dissociative forces.
False
Suppose that an angry bird of mass 1 kg, flies through the air at a speed of 100 miles per hour, and that at
some point, the bird hits a (presumably happy) green pig of mass 99 kilograms, who had, until that time, been
stationary. Suppose further, that the bird sticks to the pig. What is the speed of the pig and the bird together
immediately after the collision?
1 mile per hour
Ultimately, the principle of conservation of momentum can be linked to Newton’s third law. Since the forces
acting between objects must be equal and opposite, then for a give interaction time, the momentum change in
one object must be equal an opposite to the momentum change in the other.
True
Momentum is defined to be mass x velocity. In the absence of external forces, the mass x velocity of any object
cannot change. This is a consequence of the law of conservation of momentum.
True
A dog is laying on the ground minding its own business when a cat walks by. The dog bounds up and gives
chase. Before that cat came, there dog had no motion. How does the principle of conservation of momentum
apply?
The dog’s motion came from the Earth, and the Earth itself lost and equal amount of motion to what the
dog gained. You can say the Earth recoiled from the bounding dog!
In the energy video, what did Prof. Goodstein call the “true” energy crisis?
Each time energy is changed from one form to another, the quality of that energy degrades, making
energy eventually useless to us.
Kinetic energy is defined as
1/2 x mass x velocity squared
Suppose that one person throws a shot put at 10 meters a second, and another puts it 20 meters a second. How
many times more kinetic energy does the second put than the first?
four times as much
A weight lifter does work to lift a weight and gives the weight
gravitational potential energy
Regarding the principle of conservation of energy and the conservation of kinetic and potential energy it is
general true that
No one form of energy is “conserved” by itself, and that invariably kinetic energy is converted to thermal
or heat energy.
In the Momentum video the idea of the “Mechanical Universe” is attributed to
Rene Descartes
In the Momentum video we learn that when collisions are perfectly elastic between two equal sized balls billiard
balls, so that both kinetic energy and momentum are conserved at the same time, and when one of them is
initially at rest, then there are only two possible outcomes.
Either one ball must stop while the other goes on, or they must come away at a 90 degree angle.
In the Momentum video, the study of the structure of atomic nuclei by colliding or smashing particles together
while applying the law of conservation of momentum to look at the products is compared to
studying music by listening to a piano fall downstairs
Refering to the Momentum video: Give one good reason for Giordono Bruno’s being burned at the stake!
He was a Copernican!
In the Energy video, the question is asked of the final disposition of kinetic and potential energy, “Where did it
go”, the answer was, “It hit the dirt”, and the subsequent discussion explained that this meant
friction ultimately dissipates it as waste heat.
In th energy video, work is defined as product of force exerted through a distance. In the case of weight lifted,
the work done by the person lifting a weight is described by
Weight times height
According to the Energy video the sequence of energy events for a pole vaulter is
the kinetic energy of a runner is transferred to the elastic potential energy of a pole and then to the
gravitational potential energy of the vaulter and then to the kinetic energy of the falling vaulter.
One simple way to define the concept of Entropy is to say
Entropy is a measure of the quality of energy. The higher the quality of the energy the lower the entropy.
Energy can often be easily converted from one form to another. For instance the chemical energy in natural gas
can be converted to the thermal energy of hot, highly pressurized steam. The thermal energy of the steam can
be turned in to the mechanical energy of a spinning turbine. The kinetic energy of a spinning turbine can be
converted into the electrical energy produced in a generator connected to the turbine. The electrical energy
from a power plant’s generator can be converted into the thermal energy in the heating element of your kitchen
toaster, and thus with electrically toasted bread, we have the chemical energy that fuels civilization. Considering
the entire process, every bit of energy that comes from the burning of the natural gas can be accounted for.
None is lost. But something changes. What statement best describes this?
Although energy is strictly conserved, at each conversion from one form to another the entropy of the
energy increases.
The energy of the gas, like air, is simple related to his temperature. If you took all the air in a room and put it in
a compressed air tank at the same temperature it had in the room, it would have exactly the same amount of
energy as when it was in the room. If it is the same amount of energy in both cases, what is different?
The entropy of the gas in the compressed air tank is much lower reflecting the improbability of finding
the gas in that confined space
In order to make “heat run up hill” from cold to hot, and thus lower the local entropy of where this happens,
what must be done?
Work must be injected into the system.
In an otherwise sealed kitchen, the refrigerator door is left open. What should happen?
The refrigerator is a room heater with or without the door open. Its job is to take heat from the food in
the box and dump it in the kitchen. To do this, work is done (in the compressor) and this energy is
converted to waste heat that is also dumped into the kitchen. In leaving the door open, the compressor
would run continuously trying to cool the room, dumping ever more waste heat into the kitchen.
One of the consequences of the second law of thermodynamics to come from Sadi Carnot’s work is the principle
that
In order to make a heat engine more powerful it would necessarily be less efficient owing to the
requirement of needing to exhaust more heat.
A toy engine was shown in class. It illustrated the idea the energy can be converted from one form to another.
Which sequence do you suppose represents that illustrated in the demonstration best:
Chemical to thermal. Thermal to mechanical. Mechanical to electric. Electric to radiant.
Entropy is a measure of the number of possible arrangements of matter and energy that give a particular “state”
or configuration of nature at that we see.
True
To say that nature “Always rolls a seven” is to say that the principle of the 2nd law of thermodynamics is that we
will always find nature in its most likely, probabilistic state.
True
You hear a rattling, clanking noise in your car. You decide to keep driving it, because, after all, the part that is
out of place might just as easily jiggle itself back into place as not… You are just like my genius level, high
functioning brilliant daughters that I otherwise love and admire very much, but who do not seem to appreciate
the 2nd law of thermodynamics as much as they should.
True
