UniversityPhysicsVolume3-Ch05. Collin County Community College District GOVT 2306

Unit 2: Modern Physics
Chapter 5: Relativity
University Physics Volume III
Unit 2: Modern Physics
Chapter 5: Relativity
1. Which of Einstein’s postulates of special relativity includes a concept that does not fit with the
ideas of classical physics? Explain.
Solution
the second postulate, involving the speed of light; classical physics already included the idea that
the laws of mechanics, at least, were the same in all inertial frames, but the velocity of a light
pulse was different in different frames moving with respect to each other
3. When you are flying in a commercial jet, it may appear to you that the airplane is stationary
and Earth is moving beneath you. Is this point of view valid? Discuss briefly.
Solution
yes, provided the plane is flying at constant velocity relative to the Earth; in that case, an object
with no force acting on it within the plane has no change in velocity relative to the plane and no
change in velocity relative to the Earth; both the plane and the ground are inertial frames for
describing the motion of the object
5. To whom does the elapsed time for a process seem to be longer, an observer moving relative to
the process or an observer moving with the process? Which observer measures the interval of
proper time?
Solution
The observer moving with the process sees its interval of proper time, which is the shortest seen
by any observer.
7. To whom does an object seem greater in length, an observer moving with the object or an
observer moving relative to the object? Which observer measures the object’s proper length?
Solution
The length of an object is greatest to an observer who is moving with the object, and therefore
measures its proper length.
9. Suppose an astronaut is moving relative to Earth at a significant fraction of the speed of light.
(a) Does he observe the rate of his clocks to have slowed? (b) What change in the rate of
earthbound clocks does he see? (c) Does his ship seem to him to shorten? (d) What about the
distance between two stars that lie in the direction of his motion? (e) Do he and an earthbound
observer agree on his velocity relative to Earth?