SEPTEMBER 3, 2013
Reminders
• To the teacher: Turn on the recording!
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• To students:
• From Mr. Lee and Ms. Betz: Log in to my.ncssm.edu to retrieve
your NCSSM mail. You can have multiple Gmail accounts on your
smartphones and can login multiple Gmail accounts by using two
browsers or multiple Chrome users in Chrome settings.
• Your proctor info (C03) is due tomorrow. Email me today if you’re
having trouble finding a proctor.
• Make sure your scanner is working and available. Scanned
assignments coming up include L103D (tomorrow), P101
(Thursday), P103-prob.1 (Sunday), P103-probs.2,3 (Monday), L103
report (Tuesday next). Scan multiple pages to a single file. Make
sure the pages are arranged in order and in the correct orientation
for reading! (The only exception is your hand-drawn graph in L103.
This will be landscape orientation.) Name the file according to the
course convention.
Question: How can you quickly find
information about requirements for
submitting assignments?
Question: How can you quickly find
information about requirements for
submitting assignments?
See the home page menu, far left.
Question: What to do when you miss an
assignment deadline?
Question: What to do when you miss an
assignment deadline?
1. For WebAssign, request an extension through
WebAssign.
2. For BrainHoney, email me your extension request.
3. Complete and submit the assignment as soon as
possible but no more than 1 week after the due date.
4. While you’ll receive a late penalty on the assignment
itself, you can avoid a Commitment deduction.
For complete information, see the Course Info link in the
home page menu.
A New Location for the Lab & Problem
Forum
Students have been emailing me with questions—sometimes the same
question from multiple students. That’s been ok in the absence of a
working Lab & Problem Forum. Now there’s a working one here:
http://moodle.ncssm.edu
Log in with the same account credentials that you use for NCSSM mail.
You’re already enrolled in the course entitled Online AP Physics. You
can access it from the Navigation pane at upper left. Click on My
courses.
You should use this forum for questions about assignments, and you
should also answer questions that you can.
Plan ahead, particularly for lab work
• If you need equipment, make sure you have your equipment when
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you need it. To find out what equipment you need, open the lab and
read the equipment list.
You typically have 1-week lead time on lab reports. Take advantage of
it. One days when a lab report is due, there’s usually something else
due, too. So if you what to the last minute to do the lab, the quality of
your work will suffer.
Today was an example. Your L101 report and L103 prelab were due.
Next Tuesday, your L103 report and the L105 prelab are due.
Regarding L103, you need to take your data and submit your original
data page tomorrow as assignment L103D.
Regarding L105, during next week’s WebEx session, you’ll collect
data. Therefore, you’ll need to have your equipment set up near the
computer where you tune in.
Review of Scaling
Here are some scaling relationships that you studied and used last
week:
1.Area scales as the square of the scale factor.
2.Volume scales as the cube of the scale factor.
3.Weight scales the same as volume when density is constant.
4.Breaking strength scales as the area of cross section.
5.Relative strength scales inversely as the scale factor. Since weight
increases as the cube of the scale factor while muscle and bone
strength increase as the square of the scale factor, weight increases
much faster than muscle and bone strength. For this reason, larger
animals generally have thicker legs in relation to the size of their
bodies.
Question: A typical elephant weighing 5 - 7 tons
supports itself on very thick legs. How could an
animal that weighed 100 tons support itself?
Question: A typical elephant weighing 5 - 7 tons
supports itself on very thick legs. How could an
animal that weighed 100 tons support itself?
If you’re a whale, you swim. No legs necessary.
Extending the Discussion
Similar arguments apply to the stresses that are applied to the body in
other situations. Humans can drop from a height of 2 m onto their feet
without injury (but be sure to bend your knees!). If an elephant were
dropped that distance, its legs would be crushed. A mouse, on the other
hand, can fall many stories without injury.
With a mouse, there’s another factor at work
that helps the mouse avoid injury in a fall from a
long distance. What is that factor?
With a mouse, there’s another factor at work
that helps the mouse avoid injury in a fall from a
long distance. What is that factor?
Air resistance
The force of air resistance scales as cross-sectional area of the falling
object. This force acts opposite the direction of motion and serves to
prevent the mouse from reaching a high enough speed to hurt itself.
Consider another force: Surface Tension
Some insects can walk on water. Humans can't. The force that supports
an insect is surface tension, and this force scales as the surface area
that is in contact with the water. Thus, surface tension scales in the
same way as bone strength. For this reason, water can more easily
support insects than people.
What if the insect falls into water? Does it still have an advantage over
humans? Why or why not?
Consider another force: Surface Tension
Some insects can walk on water. Humans can't. The force that supports
an insect is surface tension, and this force scales as the surface area
that is in contact with the water. Thus, surface tension scales in the
same way as bone sIf, however, the insect falls into the water, it will
most likely die. In that case, the surface tension becomes a formidable
force trapping the insect in a layer of water. A human, on the other
hand, could just shake it off.trength. For this reason, water can more
easily support insects than people.
What if the insect falls into water? Does it still have an advantage over
humans? Why or why not?
The insect will most likely die. The surface tension becomes a
formidable force trapping the insect in a layer of water. A human, on the
other hand, could just shake it off.
A Scaling Problem
Sphere A has 10 times the diameter as Sphere B. If the spheres have
the same composition, how does the ratio of Volume to Surface Area
for Sphere A compare to that for Sphere B?
Send me a private chat message with your numerical result only.
A Scaling Problem
Sphere A has 10 times the diameter as Sphere B. If the spheres have
the same composition, how does the ratio of Volume to Surface Area
for Sphere A compare to that for Sphere B?
Volume scales as the cube of the diameter while surface area scales as
the square of the diameter. Therefore, the ratio of volume to surface
area (V/A) scales as d3/d2 = d1.
Back to elephants—elephant ears, that is. What
purpose does an elephant’s large ears serve?
Back to elephants—elephant ears, that is. What
purpose does an elephant’s large ears serve?
They help cool the animal. The rate of cooling through evaporation of
water from the surface is proportional to the surface area of the ears.
Why don't smaller animals require such oversized ears? Consider the
previous result that the V/A ratio of spheres scales as the diameter.
Then a spherical elephant a thousand times heavier than a spherical
human would have a V/A ratio 10 times greater.
Most of the mass of the animal produces thermal energy through
normal biological processes, but there's only one-tenth the surface
area, in relation to that of a human, to provide evaporative cooling. Now
stick 2 large, thin disks on the sphere. The disks, due to their shape,
maximize the amount of surface area for their weight. They add very
little to the overall weight of the sphere while adding greatly to the
surface area. Thus, the efficiency of cooling is greatly increased.
Scaling Discussions: D103
Next you’ll spend about 10 minutes on BrainHoney participating in
discussions similar to the previous ones.
There are 5 questions. The class will be divided into small groups to
discuss answers to the questions. Here are the groups:
Question 1: Coletti, Connor, Cook
Question 2: Dalla Rosa, Fogg, Holliday
Question 3: Hwang, Lei, Matrejek
Question 4: McKinney, Nguyen, Roush, Schumak
Question 5: Simpson, Stephens, Tian, Wu
Post responses only to your own discussion thread. Don’t start new
threads. The goal is to come up with a scaling argument to answer the
question. After 10 minutes of discussion, we’ll reconvene as a class,
and I’ll ask a representative of each group to summarize the
discussion.