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WS#4: Energy & General Relativity
#1-6: True for False. If false, explain why.
1. The first postulate of the theory of special relativity is that the laws of nature are the same in
all uniformly moving reference frames.
2. The second postulate of the theory of relativity is that the speed of light in empty space
depends on the motion of either the source or the observer.
3. Just as it is possible to travel in a spaceship and come back to earth at a time far in the future,
it is also possible to travel in a spaceship and come back to earth at some time in the past.
4. The sun’s mass is decreasing.
5. Mass and energy are equivalent.
6. According to the general theory of relativity, it is possible to reverse time and grow younger
when you are near a black hole.
7. How much energy is contained in a 0.004 kg penny?
8. In a typical nuclear reactor, Uranium-236 decays into Krypton-92 and Barium-141. In this
reaction about 0.1% of the original Uranium turns into energy.
a. How much energy is produced when 0.000236 kg of Uranium-236 decays?
b. If one person uses 1.8x107 J of energy in one day, how many people could this
reaction support for one day?
9. Fusion reactions occur in the sun’s interior causing the sun to give off the energy by emitting
electromagnetic radiation. Its total power output if 3.92x1026 W.
a. How much mass is the sun loosing each second?
b. How much mass is the sun loosing each year?
c. The sun has a mass of 1.99x1030 kg. What ratio of the sun’s mass is turned into energy
each year? (Mlost/Msun)
10. In a spaceship accelerating at g, far from earth gravity, you do your maximum number of
push-ups. How does this number compare to the number you can normally do here on
earth?
11. Exactly what is equivalent in the principle of equivalence?
12. An astronaut awakes in her closed capsule, which actually sits on the moon. Can she tell
whether her weight is the result of gravitation or of accelerated motion? Explain.
13. Should a person who is worried about growing old live at the top or the bottom of a tall
apartment building? Why?
14. What is the difference between special relativity and general relativity?
15. What happens to light as it approaches a strong gravitational field?