Help with Cos and Sin

1. Dec 3, 2007

chemboy

Im having an issue of when to use Cos and Sin.

When calculating Force (Normal and Frictional) with multiple demension problems you are typically given the angle of the incline.

When trying to calculate for Fgx (or basically x) I would think to use cos(theta) and when dealing with Fgy (or basically y) I would think to use sin(theta).

Problem is my trig is 13 years rusty, and I really only need to know enough to work with these physics problems. Where I thought the above was correct, the text book had two examples and x and y in one question was calculated using cos and sin (for x and y respectively) and then the other question sin and cose where used to solve for (x and y).

Its throwing me through hoops now (maybe the difference was the shape of the triangle).

Any direction or help is greatly appreciated.

Thanks

2. Dec 3, 2007

chemboy

The question is about a skateboarder going down an incline so the book draws the triangle as a left angled triangle in the second example (if this is the proper name for this), so does this reverse the laws of cos and sin??

3. Dec 3, 2007

Gokul43201

Staff Emeritus
It all depends on which of the triangle's acute angles is labeled theta. In general, you need to remember that the sine is the ratio of opposite side/hypotenuse and the cosine is the ratio of the adjacent side/hypotenuse. The adjacent and opposite sides are identified with respect to the angle labeled as theta. With this knowledge you will always be able to determine when to use which function, and simply changing the labels will not throw you for a loop.

4. Dec 3, 2007