Physics 215 Test #1
Feb. 1, 2007
Instructor: Julie Alexander
Please write your answers on the paper provided. Write on ONE side only and leave at
least 4 lines blank between questions. Show all of your work.
1) Determine the amplitude of the resultant wave when two sinusoidal waves having the
same frequency and traveling in the same direction are combined, if their amplitudes
are 3.70 cm and 5.12 cm and they differ in phase by π/2 rad.
sin(A+B)=sinAcosB+cosAsinB
2) In an experiment on standing waves, a string 92.4 cm long is attached to the prong of
an electrically driven tuning fork, which vibrates perpendicular to the length of the
string at a frequency of 60.0 Hz. The mass of the string is 44.2 g. How much tension
must the string be under (weights are attached to the other end) if it is to vibrate with
four loops?
3) An experimenter arranges to trigger two flashbulbs simultaneously, a blue flash
located at the origin of his reference frame and a red flash at x=30.4 km. A second
observer, moving at a speed of 0.247c in the direction of increasing x, also views the
flashes.
a) What time interval between them does she find?
b) Which flash does she say occurs first?
4) It is concluded from measurements of the red shift of the emitted light that quasar Q1
is moving away from us at a speed of 0.788c. Quasar Q2, which lies in the same
direction in space but is closer to us, is moving away from us at speed 0.413c. What
velocity for Q2 would be measured by an observer on Q1?
5) Consider the following, all moving in free space: a 2.0 eV photon, a 0.40 MeV
electron, and a 10 MeV proton.
a) Find the speeds of each. Which is moving the fastest?
(The rest mass of an electron is 0.511 MeV/c2 and of a proton is 938 MeV/c2)
6) UV light of wavelength 350 nm and intensity of 1 W/m2 is directed at potassium that
has a work function of 2.2 eV.
a) Find the maximum kinetic energy of the photoelectrons.
b) If 0.5% of the incident photons produce electrons how many electrons per second
are emitted from a 1 cm2 surface area of the material?
1eV=1.6X10-19J, c=3X108 m/s, h=6.626X10-34Js
Answers: Each question was worth 5 marks.
1) 6.32 cm
2) 36.7 N
3) (a) -2.58 X 10-5 s
(b) red
4) -0.556c
5) photon v=c
electron v=0.827c
proton v=0.145c
6) (a) 1.34 eV
(b) 8.8X1011 electrons/s
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