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

v x r

• 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