Lecture 3

1. Lecture 3 Ast 1001 6/4/07

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. Conservation Laws • Conservation of linear momentum • Conservation of orbital angular momentum • Angular momentum = m x v x r • Rotational angular momentum • Conservation of energy

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

16. Conservation of Energy • The amount of energy (in all forms) remains perfectly constant • Lots of examples • Balls • Pendulums • Universe as a whole

17. 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.

18. 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

19. 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

20. Escape Velocity • If we give something enough energy, it will go up, but not come down

21. 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

22. 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

23. 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

24. Light and Matter • Emission • Absorption • Materials that absorb light are called opaque • Transmission • Materials that transmit light are called transparent • Reflection/Scattering

25. 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

26. 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

27. 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)

28. 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)

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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