Physics on the Back of an Envelope

Here is the master list of questions
 

NEWS and Homework:

FINAL EXAM: FRIDAY MAY 3, 8:30-11:30, OCNPS 202

Homework due 4/10:
#99: A biologist recently claimed to have revived a 30 million year old bacterium.  How many cosmic rays would have passed through the nucleus of the bacteria during the last 30 million years?  What is the probability that its DNA is scrambled?  Approximately 1 cosmic ray passes through your hand every second (typically a high energy muon).
#101: Someone places 1 Curie of Uranium on your chest.  What do you die of?  [A curie is a unit of radioactivity.  It is 3.7 * 10^10 disintegrations per  second.  In the case of uranium, a disintegration would be a nuclear fission.  In the case of 14C, a disintegration would be a beta-decay to 14N.  The half life of uranium is comparable to the age of the Earth (since it has not all decayed yet).  You will need to calculate your radiation dose.  Dose is measured in rads.  1 rad means an energy deposition in your body by the radiation of 0.01 J/kg.  A dose of 1000 rads is fatal to humans.]

Homework due 4/3:
How much uranium does a 1 GW electric nuclear power plant require per year? One uranium atom releases 100 MeV of energy when fissioned.
How much land does a 1 GW electric solar power plant require?
How much land does a 1 GW electric windmill power plant require?

The midterm exam will be Wednesday March 6.  A sample midterm is here.

Homework due 2/27:
  #42.5: Compare the tidal force on the Earth of the Sun and the Moon.
  #32: This question is actually several questions:

1. At what distance from the Earth is geostationary orbit?  (ie: at what altitude will a satellite orbit with a period of one day so that it stays over the same point on the Earth's surface?)
2. If a cable hung down to Earth from that point, how much would it weigh?  Ignore all orbital mechanics but do include the change in Earth's gravity with altitude.  Make reasonable approximations.  Assume that the cable material is solid (ie: does not include much air [eg: not foam or aerogel]). Remember that water has a density of 1000 kg/m^3 and that very few solids are much lighter than that (unless they are mostly air).  You can find the densities of common materials in your intro physics text.  If you sold that textbook (shame on you), there are copies of texts in the Learning Center.
3. What would the tensile stress be on this cable (ie: the weight per unit area)?
4. How does this stress compare with the tensile strength of normal materials (eg: steel, kevlar, spider silk [you choose])?  You can find the tensile strength of many materials in your intro physics text.
5. We will discuss the ultimate theoretical tensile strength of materials in class next week.