Homework Help: Finding direcitons of vectors - MIdterm is tomorrow.

1. Jan 28, 2008

1. The problem statement, all variables and given/known data

Im wondering about applying trigonometry to physics. Using the tangent, sin, and cosine stuff. WHen looking at a vector that makes a right triangle, how do I know when to use tangent,sin, or cosine?
I know cosine relates to the X and Sine relates to the Y. But how do I know about the others

2. Relevant equations

3. The attempt at a solution

2. Jan 28, 2008

PowerIso

I think part of your problem is you think x relates cosine and y relates to sine. That's only true when your reference angle says it is true.

I think the simple idea is that you have an angle and you form a right triangle. You then refer to the side directly across the triangle to be your velocity. Depending on where your angle is drawn you can infer some more information. For example, if your angle is drawn at the origin under the x = y line, then you have

Cosine = x/v
Sine = y/v

Analysis that. Since Cosine is your adjacent side over your hypotenuse, you see that x/v is cosine. Same logic for sine.

If your angle is above the x = y line then you can see that the angle adjacent to theta would be y not x, so now cosine = y/v and sine = x/v.

To use tangent would be used if you need to decide what your reference angle is, so you would use side adjacent to the angle over the side opposite of the angle, whatever that may be.

3. Jan 28, 2008