PHYS 252 TEST #4 Dr. Holmes 3/28/08 NAME
ANSWER ALL 8 QUESTIONS. THE WORTH OF EACH QUESTION IS AS MARKED. SHOW YOUR WORK FOR PARTIAL CREDIT.
1) a) What was the purpose of the Michelson-Morley experiment?
b) Describe the experimental setup for the Michelson-Morley experiment and explain the basic idea on which the experiment is based.
c) What was the result of the experiment?
2) a) Distinguish a proper length from a non-proper length:
b) Give one example (in a real or in a "thought" experiment) of a non-proper length measurement; be sure to say what is being measured and by whom:
c) Is the proper length: always smaller than, always larger than, sometimes larger than, or always the same as the non-proper length?
Proper length is always longer than a non-proper length.
3) a) Why is c the upper limit for the speed of material objects?
b) If c is the upper speed limit, explain why it is possible for a person who lives less than 100 years to travel distances greater than the 100 light years (assuming the person has a space ship that can go fast enough).
4) Describe TWO REAL (not just thought) experiments (other than the Michelson-Morley experiment) that:
(a) can be explained by special relativity but cannot be explained by the classical theory;
(b) tell how classical theory fails in each case; and
(c) how relativity succeeds in each case:
5) Two explosions (call them # and $) are seen by observer A: the $ explosion happens 1,100 meters to the left of the # explosion, and the $ happens 4 microseconds before the # explosion. Observer B is moving with a speed of .881c to the right with respect to the A observer.
a) What does observer B measure for the distance between the two explosions?
90.5 meters
b) Did the $ explosion happen to the right or left of explosion # according to observer B?
Left.
c) What does observer B measure for the time difference between the two explosions
1.63 microseconds.
d) Which explosion happened first (# or $) as determined by observer B?
$ .
6) a) How fast would a spaceship have to travel for the personnel on the spaceship to age 1 year on the spaceship while the earth observers say the ship has been in flight for 18 years as determined by the earth?
.9985 c.
b) Which of these two times (the 1 year or the 18 years) is the proper time for the age of the personnel?
1 year.
c) According to the earth observer, how far will the spaceship have traveled during the 18 years?
17.9722 light years.
d) According to the spaceship observer, how far will the spaceship have receded from the earth during the 1 year?
.9985 light years.
e) Which of these two distances (answer to c or answer to d) is the proper length of that section of the galaxy?
17.97 light years (answer c).
7) A space ship moving at a speed of .78c away from the earth (as measured by both the earth and the spaceship) fires a missile going toward the earth at a speed of .96c relative to the space ship.
a) What speed would an earth observer measure for the missile?
0.717 c .
b) Is the missile going toward the earth or away from the earth?
Toward the earth.
c) If the missile were fired away from the earth instead of toward it, what speed would the earth observer measure for the missile?
0.995 c.
d) Is the missile now going toward the earth or away from the earth?
Away from the earth.
e) If the missile were replaced by a light pulse directed toward the earth, what speed would the earth measure for the speed of the light pulse?
1.00 c.
8) A photon has energy 2.43 MeV.
a) What is the wavelength of light that has photons of this energy?
5.12 x 10-13 m
b) What type of light is this (IR, UV, radio, etc.; if visible, what color is it)?
x-rays or gamma rays.
c) What is the mass of this photon?
4.32 x 10-30 kg.
d) What is the rest mass of this photon?
Zero kilograms.
e) What is the momentum of this photon?
1.295 x 10-21 kg*m/s .
f) If we solved for the speed of an electron with this same momentum, does the classical momentum, p = mov , give an answer accurate to within 10% (yes or no)?
No.
g) If an electron had a kinetic energy of 2.43 MeV, what speed would it have?
0.985 c.
h) If we solved for the momentum of this electron, does the classical momentum, p = mov , give an answer accurate to within 10% (yes or no)?
No.