Transcript Unit 1

Homework 3
Unit 18 Problem 10, 12, 13(only b), 17, 18, 20
Unit 19 Problem 17, 20
Escape Velocity is for more
than just Rockets!
• The concept of escape velocity is useful
for more than just rockets!
• It helps determine which planets have an
atmosphere, and which don’t
– Object with a smaller mass (such as the
Moon, or Mercury) have a low escape
velocity. Gas particles near the planet can
escape easily, so these bodies don’t have
much of an atmosphere.
– Planets with a high mass, such as Jupiter,
have very high escape velocities, so gas
particles have a difficult time escaping.
Massive planets tend to have thick
atmospheres.
The Origin of Tides
• The Moon exerts a
gravitational force
on the Earth,
stretching it!
– Water responds to
this pull by
flowing towards
the source of the
force, creating
tidal bulges both
beneath the Moon
and on the
opposite side of
the Earth
High and Low Tides
As the Earth rotates beneath
the Moon, the surface of the
Earth experiences high
and low tides
The Sun creates tides, too!
•
•
The Sun is much more massive than the
Moon, so one might think it would create
far larger tides!
The Sun is much farther away, so its tidal
forces are smaller, but still noticeable!
•
•
When the Sun and the Moon line up,
higher tides, call “spring tides” are formed
When the Sun and the Moon are at right
angles to each other, their tidal forces work
against each other, and smaller “neap
tides” result.
The Conservation of Energy
• The energy in a closed system may change
form, but the total amount of energy does not
change as a result of any process
Kinetic Energy
• Kinetic Energy is simply the energy of
motion
• Both mass (m) and velocity (V) contribute
to kinetic energy
1
2
EK = m ´ V
2
• Imagine catching a thrown ball.
– If the ball is thrown gently, it hits your hand
with very little pain
– If the ball is thrown very hard, it hurts to
catch!
Thermal Energy
• Thermal energy is the energy
associated with heat
• It is the energy of the random motion
of individual atoms within an object.
• What you perceive as heat on a
stovetop is the energy of the individual
atoms in the heating element striking
your finger
Potential Energy
• You can think of potential
energy as stored energy,
energy ready to be converted
into another form
• Gravitational potential energy
is the energy stored as a result
of an object being lifted
upwards against the pull of
gravity
• Potential energy is released
when the object is put into
motion, or allowed to fall.
Conversion of Potential Energy
• Example:
– A bowling ball is lifted from the floor
onto a table
• Converts chemical energy in your
muscles into potential energy of the ball
– The ball is allowed to roll off the table
• As the ball accelerates downward
toward the floor, gravitational potential
energy is converted to kinetic energy
– When the ball hits the floor, it makes a
sound, and the floor trembles
• Kinetic energy of the ball is converted
into sound energy in the air and floor, as
well as some heat energy as the atoms
in the floor and ball get knocked around
by the impact
Definition of Angular Momentum
• Angular momentum is the rotational equivalent of
inertia
• Can be expressed mathematically as the product of the
objects mass, rotational velocity, and radius
• If no external forces are acting on an object, then its
angular momentum is conserved, or a constant:
L = m ´V ´ r = constant
Conservation of Angular Momentum
• Since angular momentum is
conserved, if either the mass,
size or speed of a spinning
object changes, the other
values must change to
maintain the same value of
momentum
– As a spinning figure skater
pulls her arms inward, she
changes her value of r in
angular momentum.
– Mass cannot increase, so her
rotational speed must increase
to maintain a constant angular
momentum
• Works for stars, planets
orbiting the Sun, and satellites
orbiting the Earth, too!
The Nature of Light
• Light is radiant energy.
• Travels very fast –
300,000 km/sec!
• Can be described either
as a wave or as a
particle traveling
through space.
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•
As a wave…
– A small disturbance in an electric field creates
a small magnetic field, which in turn creates a
small electric field, and so on…
• Light propagates itself “by its bootstraps!”
– Light waves can interfere with other light
waves, canceling or amplifying them!
– The color of light is determined by its
wavelength.
As a particle…
– Particles of light (photons) travel
through space.
– These photons have very specific
energies. that is, light is quantized.
– Photons strike your eye (or other
sensors) like a very small bullet, and
are detected.
The Effect of Distance on Light
• Light from distant objects
seems very dim
– Why? Is it because the photons
are losing energy?
– No – the light is simply
spreading out as it travels from
its source to its destination
– The farther from the source you
are, the dimmer the light seems
– We say that the object’s
brightness, or amount of light
received from a source, is
decreasing
Brightness =
Total Light Output
4pd 2
This is an inverse-square law –
the brightness decreases as the
square of the distance (d) from
the source
The Nature of Matter
• The atom has a nucleus at
its center containing protons
and neutrons
• Outside of the nucleus,
electrons whiz around in
clouds called orbitals
– Electrons can also be
described using wave or
particle models
– Electron orbitals are quantized
– that is, they exist only at
very particular energies
•
– The lowest energy orbital is
called the ground state, one
electron wave long
•
To move an electron from one orbital to the next higher
one, a specific amount of energy must be added.
Likewise, a specific amount of energy must be released
for an electron to move to a lower orbital
These are called electronic transitions
The Chemical Elements
• The number of protons (atomic number) in a nucleus
determines what element a substance is.
• Each element has a number of electrons equal to the
number of protons
• The electron orbitals are different for each element,
and the energy differences between the orbitals are
unique as well.
• This means that if we can detect the energy emitted or
absorbed by an atom during an electronic transition,
we can tell what element the atom belongs to, even
from millions of light years away!
Measuring Temperature
•
It is useful to think of temperature
in a slightly different way than we
are accustomed to
– Temperature is a measure of the
motion of atoms in an object
– Objects with low temperatures have
atoms that are not moving much
– Objects with high temperatures have
atoms that are moving around very
rapidly
•
The Kelvin temperature scale was
designed to reflect this
– 0  K is absolute zero –the atoms in
an object are not moving at all!
Results of More Collisions
• Additional collisions mean
that more photons are
emitted, so the object gets
brighter
• Additional hard collisions
means that more photons of
higher energy are emitted, so
the object appears to shift in
color from red, to orange, to
yellow, and so on.
• Of course we have a Law to
describe this…
Wien’s Law and the Stefan-Boltzmann Law
• Wien’s Law:
– Hotter bodies emit more
strongly at shorter
wavelengths
• SB Law:
– The luminosity of a hot
body rises rapidly with
temperature