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PHY138Y1-Y |
Waves Quarter – Written Team Problem Set |
Prof. Jason Harlow |
|
“Physics for
the Life
Sciences I” |
Due |
Physics Department, |
Please
complete the following problems on separate paper. You should solve the problems together in the
same team that you have been working with for the Waves Quarter in your tutorials.
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 on time.
Make sure
the following information appears clearly at the top of the first page of your
submitted copy of the solutions:
- All team member’s names
- Your tutor’s name
- Your tutorial section day and time
Show all
your reasoning and work legibly, and draw a box around the final numerical or
single-word answer where applicable.
Your solutions should be stapled and submitted in the Drop Box for your tutorial. The Drop Boxes are located in the basement of
the Burton Tower of McLennan Physics Labs (MP).
On the first floor of MP there is a stairway with a bust of Isaac Newton
on the 1st floor; the Drop Boxes are at the bottom of the stairs. This
assignment is due by
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 W, and the irradiating area of the ultrasound-emitting head is A = 6 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):