Physics on the Back of an Envelope (original) (raw)

Physics 309 Spring 2008

Room 303 OCNPS, W 10-11
L. Weinstein
683 5803
Room 217 Oceanography and Physics Building
Room 104 Nuclear and Particle Research Facility, 1021 W 47th St, Norfolk, VA 23529
Office Hours: Wed 11-12 Learning Center and by apppointment
weinstei -at- physics -dot- odu -dot- edu

Physicists should be able to estimate the order-of-magnitude of anything. How many atoms of Julius Caesar do you inhale with each breath? How much waste does a nuclear power plant generate? This 1 credit course will develop concepts, relations and numbers useful for estimation. We will discuss the concepts as a group and attack the problems as a group. I intend to lecture as little as possible. The course will not cover new material but will make use of already acquired (or at least already taught) knowledge. It will try to help students apply physics to real-life questions and understand which physical effects are appropriate on which scales. The corequisite is Physics 232.

Your grade in the class will depend on tests, homework and class participation.

Midterm: March 5
FINAL EXAM: Thursday, May 8, 8:30-11:30, OCNPS 303

Here is the master list of questions

Logarithmic map of the universe

NEWS and Homework:

Here are some sample tests:
2002 midterm
sample midterm

Date ProblemsSolved in Class HomeworkProblems(Due next class)
1/16/08 1,4 7,10
1/23/08 11, 12, 24, 26, 31 19, 40
1/30/08 29, 30, 34, 37(start) 36, 37
2/6/08 36, 37 45, 49, 50
2/13/08 raindrop terminal speedEarth's KE 51, 53, 56, 57 64, 67
2/20/08 54, 65, 66crater size and asteroid KEmass of air in lecture hall 71, 72
2/27/08 71, 72, 78
3/5/08 midterm
3/19/08 scaling area, volume, gravity, collisions, jump height 1) Make a 1/10 scale of bridge. Let it collapse. What speed should you film it at so it appears real? 2) How does I scale with length? 3) If you double the radius of an 8-dimensional sphere, what happens to its surface area?
3/26/2008 Dimensional analysis circle, ellipse, pulley bohr radius see below
4/2/2008 Dimensional analysis pendulum, gravitational light bending see belowContact me Monday if you need help.

Homework questions:

due 2 April:

  1. For circular orbits, use dimensional analysis to derive Kepler's third law for periods and radii.
  2. Use dimensional analysis to estimate the radius of a black hole.

due 9 April:

  1. Derive the height of the atmosphere as a function of the appropriate variables. Calculate your result numerically. Hint: the temperature, kTkTkT, should be one of your variables.
  2. What is the drag force on an object moving at speed vvv through a fluid? Calculate your result numerically for a car at highway speed.

Important numbers (memorize these):

Population of the Earth: 6*10^9
R_earth = 6*10^6 m
d_earth-sun = 1.5*10^11 m
d_earth-moon = 4*10^8 m
solar flux (at Earth orbit) = 1400 W/m^2
G = 7*10^(-11) N-m^2/kg^2
density of water = 1000 kg/m^3 = 1 kg/l = 1 g/cm^3
density of air (@stp) = 1 kg/m^3
density of iron = 10 tons/m^3
1 year = pi * 10^7 s
avogadro's number = N_A = 6*10^23
1 e = 1.6*10^(-19) Coulombs
molecular binding energy = 1.5 V
cell size = 5 * 10 ^(-6) m
atom size = 10^(-10) m

Chemistry stuff:
1 mole of gas at STP has V = 22.4 l
1 mole of gas has m (in grams) = molecular weight
specific heat of water = 1 cal/g-K = 4 J/g-K

Units:
1 m^3 = 10^3 l = 10^6 cm^3
1 ton = 10^3 kg = 10^6 g
1 atmosphere = 1 bar = 10^5 Pascal = 10^5 N/m^2 = 760 mm Hg = 10 m H2O = 15 psi
1 m/s = 2 mph (approx)

Note that the atom size can be derived from Avogadro's number and the density of water.

Lies and Damn Lies: The Art of Approximation in Science This is a great course on estimation methods, given at MIT. I plan to steal from it liberally.

Order of Magnitude Physics A high level, 3 credit, course on estimation from CalTech.

My forthcoming book on estimation for the general public.

Last modified: Wed Jan 2 13:23:49 EST 2008