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Transcript
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