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# Physics proportionality help? :)?

Topic: How to write a constant of proportionality
July 16, 2019 / By Candy
Question: Ok I have a specific question and then a much more general question.. ok, here it goes: 1) what is proportionality anyway? how is it useful? 2) using the formula F = (m . v^2)/r show that r is proportionate to T^2 [where r is radius of uniform circular motion, v is velocity of the object undergoing circular motion, m is the mass of the object undergoing circular motion and T is the period) thanks guys :)

## Best Answers: Physics proportionality help? :)?

Amelia | 1 day ago
1)Normally any proportionality condition is conditional. If we say variable A is proportional to Variable B. means keeping everything else same if A changes by a factor of k then B also changes by the same factor. Thus it is useful if change in one is given one can find the change in another. 2) F = (m . v^2)/r = [m*(2*pi*r/T)^2]/r = (2*m*pi*r)/(T^2) This equation can be written as r = [F/(2*pi*m)](T^2) So we see that if F and m are constant, we know pi is constant, r will change in the same ratio as T^2. Or we say 'r' is proportional to 'T^2'. If r doubles T^2 also doubles if r becomes one-fifth T^2 also becomes one-fifth.
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Originally Answered: Can I have my high school AP physics teacher write me a letter of recommendation for an REU in physics?
You can ask them to write one, but it's not going to look good. If you want to do an REU in physics, you should be a physics major. If that's the case, you should have been taking physics classes this year - if you placed out of the intro classes, you should be in modern, classical, or E&M at the least. With only AP physics behind you, you have a very poor shot at an REU. They are very competitive, and they look at your physics and math coursework. They also prefer it if you have done research at your own institution already. With no college level physics classes, no research at your own school, and no one to recommend you from the college level, your chances of getting into an REU are extremely poor.
Originally Answered: Can I have my high school AP physics teacher write me a letter of recommendation for an REU in physics?
Seeing as AP is supposedly equivalent to college-level, it would probably be acceptable. I would include a short note explaining the reason why you couldn't get a college professor to do it.