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# Tutor profile: Shannen P.

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Shannen P.
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## Questions

### Subject:Physics (Newtonian Mechanics)

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Question:

I can solve a normal projectile motion problem just fine, but I just got one where an object is thrown from an initial height h with a velocity v at an angle of 60 degrees, and the initial height is really tripping me up when I try to plug into the kinematic equations. How do you solve one like this?

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Shannen P.

### Subject:Physics (Electricity and Magnetism)

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Question:

How do you find the formula for the electric potential of a point above a disk?

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Shannen P.

### Subject:Biology

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Question:

My teacher told me that adenine and thymine go together, and cytosine and guanine go together... but why does this actually happen?

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Shannen P.

It has to do with the H bonds! CATG-- they're all nucleobases that make up the rungs of the DNA ladder. Like you said, when looking at the DNA of an organism, you'll find C and G in proportion to each other, and A proportional to T, but this didn't just happen for no reason. Adenine and thymine form 2 hydrogen bonds between one another, whereas cytosine and guanine form 3. Think of it like trying to plug a 3-pronged plug into a 2-pronged socket... not going to work. Same sort of idea here-- if C has 3 spots where it can form hydrogen bonds with another nucleobase, do you think it'll want to only have 2 filled? Nope, it'll want them all! Now, why is it that a base doesn't just pair with itself? That'd be a lot easier, right? Well, that ties back to a set of terms you might have heard before: purines and pyrimidines. Adenine and guanine are both purines (double rings), where as cytosine and thymine are pyrimidines (single ring). In these base pairs, purines only want to pair with pyrimidines, and vice versa, and that's how we eliminate that self-pairing possibility. Understanding the why of base pairing instead of just memorizing A is proportional to T will definitely help out further down the road of biology content. For example, if your teacher later tells you that A/T rich regions of DNA are weaker than C/G regions, you don't have to memorize the fact. Instead, by simply knowing A and T only share 2 H bonds, you can infer they'll be inherently less strong than the C and G pairs that share 3 H bonds!

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