1. Lecture 3
Ast 1001
6/4/07

2. Motion Basics Speed Velocity Acceleration
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 There are many types of energy
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 E = mc2
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 Transmission
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