PHY251 Midterm I Exam
PHY 251 Midterm I Exam
March 7
2001
I. Short Answer Questions
(110 points)
1.
List the crucial experimental evidence that showed that classical Newtonian mechanics was a low
velocity approximation of relativistic mechanics.
Solution:
(5 points)
MichelsonMorley, who showed that all electromagnetic waves (predicted by Maxwell's
equations) propagate with the same speed
c
in vacuum, regardless of inertial frame of the source
or receiver.
2.
List three types of
experimental
evidence that testify to the quantum character of the theory of
radiation.
Solution:
(9 points)
Planck's blackbody radiation, photoelectric effect, Compton scattering
3.
List two types of experimental evidence that can be understood by assuming that subatomic
particles like electrons have a wavecharacter.
Solution:
(6 points)
Electron diffraction and interference, Bragg scattering of electrons (DavissonGermer),
Tunneling, Bohr's explanation of the hydrogen spectrum.
4.
Consider a moving clock and a moving meter stick (moving parallel to its long dimension) both
speeding at constant
v
= 0.8
c
past a stationary observer (you).
a.
Calculate the length you will measure the speeding meter stick to have.
Solution:
(5 points)
g
= 1/
Ö
(10.64) = 1/0.6 = 10/6;
L
' =
L
0
/
g
= 0.6 m
b.
Calculate the time that passed on the speeding clock if a clock in your rest frame indicates
that 3.0 ns have passed.
Solution:
(5 points)
Moving clock will "tick" slower:
t'
=
gt
0
→
t
0
= 3.0 ns/
g
= 1.8 ns
c.
Indicate how you would set up the experiment for the measurement in part b) above.
Solution:
(5 points)
Several ways: set up two clocks that are perfectly synchronized, and move them
c
×3 ns =
0.30 m apart, along the direction of the speeding clock. Rig a trigger close to each of the
observers\'s clocks, so that when the moving clock passes, a nearby camera takes a
picture. Comparison of the two pictures will then show the time differences on the
speeding clock and on the observer's clocks. Note, that the observer's clocks are nor really
needed in this experiment, as long as the triggers are set up 0.30 m apart.
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 Spring '01
 Rijssenbeek
 Physics, mechanics

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