Physicists should be able to estimate the order-of-magnitude of anything.
How many atoms of Julius Caesar do you
eat every day? 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.
Here is the master list of questions
FINAL EXAM: MONDAY MAY 5, 8:30-11:30, OCNPS 202
Sample midterms are here and here. There will be no calculators or crib sheets or other external aids allowed on the test.
The antiwar protest in New York City on Feb 15 was estimated at between 1 and 4*10^5 people. If the protests filled 1st Avenue for 20 blocks, how many people were there? There are 20 NYC blocks per mile and 1st Avenue is about 6 lanes wide. Some party-poopers actually counted the crowd in San Francisco: see this link
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Homework due Jan 22: #3 (popcorn), #8 (speed of hair), #13 (human crowding). On #13, please compare the area you get to the total area of Virginia Beach.
Homework due Jan 29: #6 (cells), 31 (portapotties), 32 (fuel cost)
Homework due Feb 5: 42 (mass of moon) and 53 (1 km asteroid)
Notes on homework: #42: We determined in class that the angular size of
the moon is the same as your finger held out at arm's length (or about 1
cm at a distance of 1 m). From this we know that the moon's diameter
is 1 cm / 1m = 1% of the distance from here to the moon (which is 1/4 million
miles or 400,000 km). Please estimate the mass of the moon from its
diameter. The density of water is 1000 kg/m^3.
#53: We determined in class that a reasonable speed for an asteroid is a) the speed of a rock falling from infinity in the Earth's g field (where escape velocity is 11 km/s) or b) the speed of a rock falling from infinity in the Sun's g field (where escape velocity is sqrt(2) times larger than the Earth's orbital speed around the Sun. Since (b) is larger, use that. To get the insolation of the Earth, you need the solar constant (solar radiation flux) at Earth orbit which is about 1 kW/m^2. 1 megaton of TNT is about 10^15 J (we will derive that later).
Homework due Feb 12: #43, #48, #57
Homework due Feb 19: #57, #61
For problem 57, "how big an impact is needed to break the moon in two? to
break Phobos in
2?" has been better defined. Use the mass of Phobos = 1*10^16 kg. Assume
that Phobos consists of 2 touching spheres, each with half of the total
mass. Use the density of rock (rho_rock = 3*rho_water). Find the energy
needed to separate the two spheres to infinity (with zero kinetic energy).
Now find the size of asteroid that has that much kinetic energy. Since
we
determined in a previous problem that a 1 km^3 asteroid has a kinetic energy
of
KE = 1/2 * 10^9 m^3 * 3000 kg/m^3 * (3*10^4 m/s)^2 = 1.5*10^21 J,
we can use 10^21 J/km^3 as the kinetic energy density of a typical
asteroid. Make a reasonable assumption for the conversion
efficiency of asteroid kinetic energy to gravitational potential
energy (ie: to splitting the Moon or Phobos). Therefore, to answer
the
question "How large an impact", I want the answer in terms of the size of
the asteroid needed to hit it, ie: in km^3. Then do the same thing
for
the moon.
For problem 61, you can use 1 cm/yr for your velocity.
Homework due Feb 26: 85, 96, 106
For #85, you can use the solar constant of 1000 W/m^2 at Earth orbit (ie: at 1.5*10^8 km from the sun). Alternatively, you can use the Stephan-Boltzmann law with a surface temperature of about 5500 K.
For #96, you can use our total energy input from food or you can calculate our energy output from various forms of heat transfer.
For #106, use the energy output calculated in #96. Assume that the only form of heat output you have is perspiration (since if the ambient temperature is 98.6 F, conduction, convection and radiation are all useless).
Note that there is a discussion of this at http://quiz.thphy.uni-duesseldorf.de/98/A08.98.html
(This is a great physics problem web site: http://quiz.thphy.uni-duesseldorf.de)
March 3: Midterm Exam. No homework due. Please let me know which homework problems you want for March 17. If you don't choose some, I will.
Important numbers to be memorized for the exam:
R_earth = 6*10^6 m
d_earth-sun = 1.5*10^11 m
d_earth-moon = 4*10^8 m
G = 7*10^(-11) N-m^2/kg^2
density of water = 1000 kg/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
Note that the atom size can be derived from Avogadro's number and the density of water.
Your grade in the class will depend partially on homework and class participation. Please do both!