Midterm 1 Solutions Sp01

Modern Physics

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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) Michelson-Morley, 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 sub-atomic particles like electrons have a wave-character. Solution: (6 points) Electron diffraction and interference, Bragg scattering of electrons (Davisson-Germer), 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 (1-0.64) = 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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Midterm 1 Solutions Sp01 - PHY251 Midterm I Exam PHY 251...

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