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[Teacher-talkmissinglink] Workshop #2 on Missing Link - error?

From: Ruth Hund <ruth.hund_at_gmail.com>
Date: Sun, 12 Aug 2012 10:26:47 -0600

I think I have found a huge error on Sample C for the Carly Project. The
student says that the scale factor is 1.8. S/he does this by dividing the
height of Carly in the picture by the height of the dog in the picture. The
scale factor is a factor by which I can multiply each component in the
picture to obtain the actual height of any item in the picture. In fact,
the student does obtain the correct answer, because she is using only two
items. The student *can* set up a proper proportion, but does *not* understand
the idea of scale factor. The scale factor is the actual height of the dog,
divided by the height of the dog in the picture, which is 1.2 ft/cm. The
lead teacher also makes the error by saying that units are not used in
scale factors. Any science teacher would strongly disagree with that. It is
important to know the units - when multiplying, using dimensional analysis,
then, the student can double-check their work, by double checking their
answer. Fore example, Carly has a measured height of 4.5 cm in the picture.
Therefore, 4.5 cm * 1.2 ft/cm gives a height of 5.4 feet. The cm
conveniently divides out and leaves the student with the proper unit of
feet. The scale factor of 1.8 is dimensionless because both of 4.5 and 2.5
are in cm and divide out (4.5cm/2.5cm = 1.8). So, 4.5cm * 1.8 = 5.4 cm
which is *not* the correct answer. I would give this student a 1 or 2, not
a 3 or 4 as the teachers in the video did.

Am I not understanding something, or have I discovered an error?

email: ruth.hund_at_gmail.com
phone: 303-949-9029
Skype: hundneserke
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Received on Mon Aug 13 2012 - 08:31:48 EDT



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