Transcript Lecture 3

Lecture 3
Ast 1001
6/4/07
Motion Basics
• Speed
– How far you will travel in a given time
• Velocity
– How far you will travel in a given direction in a
given time
• Acceleration
– How much your velocity is changing in a given
time
Momentum and Forces
• Momentum is a combination of mass and velocity
– Momentum = mass x velocity
• Forces cause an acceleration
– The concept of Net Force accounts for multiple forces
at once
• Gravity is a force
– On the surface of Earth, the acceleration caused by
gravity is roughly constant (g)
Angular Momentum
• Things that are spinning
have no linear
momentum
• Any object that is
spinning or moves along
a curved path has
Angular Momentum
• You need a special kind
of force called a Torque
to change angular
momentum
Mass vs Weight
• Mass is the amount of matter in an object
• Weight describes a force (usually gravity)
that is acting on the object’s mass
• Many examples of when the difference
would be important
– Moon vs Earth
– Elevators
Free Fall
• If the elevator cable
snapped you would
be in free fall
• There is gravity in
space
• Free fall is why
astronauts are
weightless
Newton
• Aristotle claimed that
physics in the heavens
were completely different
than physics on Earth
• Newton realized that
apples fell from trees for
the same reason that
planets orbited the Sun
– This is a unification of
physics
Newton’s Laws of Motion
• An object moves at a constant velocity if
there is no net force acting upon it
– Statement of inertia
• Force = mass x acceleration
• For any force, there is always an equal and
opposite force
Conservation Laws
• Conservation of linear
momentum
• Conservation of orbital
angular momentum
– Angular momentum = m x
vxr
• Rotational angular
momentum
• Conservation of energy
Energy
• Energy is the ability to do work
– Basically, energy is what makes matter move
• There are many types of energy
–
–
–
–
Kinetic energy
Potential energy
Radiative energy
Thermal energy
Measuring Energy
• Calories are often used in food
• Joules and Ergs are used in science
• Temperature units (Degrees Fahrenheit or
Celsius, Kelvins) are used for thermal
energy
Thermal Energy
• Thermal energy is a type of kinetic energy
• Related to the motion of particles in an object
• Thermal energy and temperature are NOT the same
thing
– Thermal energy measures the total kinetic energy in
something
– Temperature measures the average kinetic energy of the
particles
The Kelvin Scale and Density
• Kelvin scale is based on
absolute zero instead of
the freezing point of
water
• How humans perceive
temperature also relies on
the density of the
material
– Hot water vs hot air
– The upper atmosphere and
astronauts
Gravitational Potential Energy
• Simply how much mass there
is and how far it can fall due to
gravity
– Higher things can fall further
• Balls moving through the air
demonstrate a transfer
between kinetic, potential
energy
• Collapsing clouds of gas
demonstrate transfer between
potential, thermal energy
Mass Energy
• Mass itself is a form of potential energy
– You can convert mass into energy
• E = mc2
– This is how the Sun gets its energy (nuclear
fusion)
Conservation of Energy
• The amount of energy (in all forms) remains
perfectly constant
• Lots of examples
– Balls
– Pendulums
– Universe as a whole
Group Work
• Use the Conservation of Energy to explain
why a pendulum moves like it does. The
two forms of energy that you care about are
kinetic and potential energy.
How Gravity Works
• Every mass attracts every
other mass through gravity
• The strength of the forces is
directly proportional to the
product of the masses of two
objects
• The strength of gravity is
proportional the to square of
the distances between the
objects
• F = G*m1*m2 / d2
Kepler and Newton Together
• Newton explained why
Kepler’s Laws worked
• Generalized the laws to
any set of objects
• Discovered unbound
orbits
• Discovered that things
orbit the Center of Mass
Escape Velocity
• If we give something enough energy, it will
go up, but not come down
Tides
• Primarily caused by the
pull of the Moon on
Earth
– Not exactly 24 hours
apart
– Causes two tidal bulges
• Sun also causes tides
– Can work with or
against the Moon’s force
More Fun with Tides
• Tidal Friction occurs because water doesn’t move
perfectly smoothly
– Causes Earth’s rotation to change very slightly
– Causes the Moon to move further and further away
from Earth
– Great example of conservation of angular momentum
– Effects are very small
• Tides are also the reason why the same face of the
Moon is always faced towards Earth
Light Basics
• Light carries radiative energy
• Power is the rate of energy flow
– Measured in units of Watts
– 1 Watt = 1 joule/s
• Light comes in various colors
– The rainbow of colors is called a
spectrum
– White light is when all of the
colors are mixed, black is the
absence of color
Light and Matter
• Emission
• Absorption
– Materials that absorb light are called opaque
• Transmission
– Materials that transmit light are called
transparent
• Reflection/Scattering
Waves
• Waves consists of peaks and troughs
• Wavelength is the distance from one peak to the next
• Frequency is the number of peaks passing by any
point each second
• Speed is how quickly the wave is moving
– Light always travels at c (300,000 meters/second)
• Wavelength x frequency = speed
What is light
• Light is both a wave and a particle
• It can be broken up into particles (or pieces)
called photons
• Each photon carries energy
– The higher the frequency (or smaller the
wavelength), the higher the energy
The Electromagnetic Spectrum
• There are many types of
light that our eyes can’t
see
• Radio waves are a kind
of light (and NOT a kind
of sound)
Group Work
• Things on everyday scales typically have
energies from around .1 joules to 1000
joules. What would the wavelength of a
photon need to be in order for that photon to
carry 1 joule of energy? (Hint: use page
152)
The Structure of Matter
• Democritus proposed that there were
particles so small, that they couldn’t be
broken down any further
– Called them atoms
• Atoms come in different types
– Correspond to different elements
Atoms
• Atoms are not indivisible
– Made up of protons, neutrons, and electrons
• The nucleus contains protons and neutrons
• Subatomic particles have charge (sometimes)
– Protons have positive charge, electrons have negative
charge, neutrons have no charge
Terminology
• Atomic Number is how many protons an
atom has
• Atomic Mass Number is how many protons
and neutrons an atom has
• Elements are defined by atomic number
– Different AMUs result in different isotopes
– 12C is “carbon 12”, 14C is “carbon 14” etc
Molecules
• Atoms can combine to form molecules
• Some molecules are simple combinations of
one element
– O2 and O3
• Compounds are combinations of 2 or more
kinds of elements
– H2O
Phases of Matter
• There are bonds in between the molecules
in a substance
• The strength of the bond dictates what state
the material is in
• Gas has the weakest bonds, Solids the
strongest bonds
Ionization
• Eventually, there is so much
energy that molecules break
apart
– This is called Molecular
Dissociation
• At even higher energies, atoms
break apart
– This is called ionization
– You can break apart the atoms to
various degrees
– A gas that has been ionized is
called a plasma