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PHY138-Y1Y |
Waves Quarter – Written Team Problem Set |
Prof. Jason Harlow |
This homework assignment is due by 5PM on Friday November 24. It should be submitted in the Drop Box
for your tutorial. The Drop Boxes are located in the basement of the
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This page must be the first page of the submitted Problem Set. |
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You must solve the problems together in the same team that you have been working with in your tutorials.
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Problem Sets done by a single individual will not be accepted. |
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By filling out and signing the form below, I certify that I took an active role in the solution of all the problems of this problem set.
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Name (Please
Print) |
Signature |
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Designate one member of your team as the coordinator. The coordinator will be responsible for assembling the final copy of your solutions and submitting them in the Drop Box on time.
The coordinator should fill out this form:
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Name of Your
PHY138 Tutor |
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Tutorial Group |
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Tutorial Day |
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Tutorial Time |
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Coordinator's
(your) Name |
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1. (15 points) The alternating pressure in
a therapeutic application of ultrasound is superimposed on the static pressure,
so that the resulting pressure changes periodically around the static pressure
value. Suppose the frequency of the
ultrasound in a treatment is f = 830
kHz, the irradiated power output is P =
15 mW, and the irradiating area of the ultrasound-emitting head is A = 6.0 cm2.
a. Calculate the
intensity of the ultrasound just below the emitter.
b. What is the
pressure amplitude and the highest value of pressure difference in the
muscle? The density of muscle is ρ= 1040 kg/m3 and the
sound velocity in it is v = 1570 m/s.
c. What is the
distance between the sites of highest and lowest pressure in the muscle?
[Note: Equation 20.34 in Knight, pg.634, states that the intensity of
any wave is given by I=CA2,
where C is some constant and A is the amplitude. In the case of sound waves, the amplitude is
the pressure amplitude above and below the static pressure in Newtons/m2. The constant is C=1/(2ρv), where ρ is the density of the material
and v is the speed of sound.]
2. (25 points) A string of fixed length L = 1.200 m is vibrated at a fixed
frequency of f = 120.0 Hz. The tension, Ts, of the string can be varied. Standing waves with
fewer than seven nodes are observed on the string when the tension is 2.654 N
and 4.147 N, but not for any intermediate tension. What is the linear density of the string?
3. (10 points) You are at a party and a
friend challenges you to measure the speed of light using only a chocolate bar
and a microwave oven. “No problem!”, you
say. You
remove the turntable from the microwave and replace with a flat, unwrapped chocolate
bar on a plate (so the plate does not rotate).
You heat it for about 20 seconds, until it just starts to melt, then
remove the chocolate bar. You notice some melted hot spots separated by cold
solid spots in the chocolate. The
distance between melted hot spots is about 6 cm. You turn the oven around and read the
manufacturer’s label which says that the frequency of the microwave lamp is 2.5
GHz. From your observations, what is the
speed of light?
4. (25 points) Knight Problem 21.68
5.
(25
points) Knight Problem 21.74
Some suggested problems for
the Waves Quarter (not to be turned in):