1. What does the acronym PET stand for? Why is PET a good example with which to begin a discussion of elementary particle physics?

3. Suppose you were traveling toward the Sun at a constant velocity of 0.25c. With what speed does the light streaming out from the Sun go past you? Explain your reasoning.

5. In own words, define what is meant by time dilation in special relativity theory. Provide a similar definition for length contraction. Give an example in which the effects of time dilation are actually observed.

7. Newton wrote: "Absolute, true, and mathematical time, of itself, and from its own Nature, flows equally without relation to anything external." Comment on the significance of this statement for two timekeepers in relative motion. In the light of special relativity, is Newton's statement valid? Explain.

9. Does E= mc ^2 apply only to objects traveling at the speed of light? Why or why not?

11. Given the Newtonian view of gravity, why is it reasonable to expect that the rate at which the universe is expanding should be decreasing with time? Do observations of the motions of remote galaxies support this position? If not, what do such observations suggest about how the rate of expansion is changing?

13. After landing on the planet Mars, two astronauts awaken from a long induced hibernation inside their windowless spacecraft. Before emerging, is there any way they can determine whether their individual body weights are the result of gravitation or accelerated motion?