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Phy 3031 Modern physics, Spring 2007
Homework set #7. Due at the start of class on Monday, February 18, 2007.
For numerical answers give two significant digits. The first should be correct, the second can
be approximate.
1. (a) Compute the rest energy, in Joules, of 1 g of dirt.
(b) If you could convert this energy entirely into electrical energy and sell it for
10 cents per kilowatt-hour, how much would you get?
(c) If you could power a 100 W light bulb with the energy, how many years would it
last?
2. Consider the following, all moving in free space: a 2.0 eV photon, an 0.40 MeV electron,
and a 10 MeV proton. The energy given is the kinetic energy of the object.
(a) Which is moving fastest?
(b) The slowest?
(c) Which has the greatest momentum?
(d) The least momentum?
3. If an electron has total energy .511 GeV, and if v is the speed of the electron, then
what is c − v?
4. When a proton and an anti-proton, at rest, annihilate each other and create two gamma
rays, what is the total energy of each individual gamma?