# Need help determining whether to use cosine or sine

1. Mar 8, 2013

### physicsnoob145

I'm having trouble understanding when to use sine or cosine in my physics problems

for example I got this from one of the lectures off youtube
and I know why they are using sine and cos because it is at an angle but I'm rotating the angle to understand why it is mgcosbeta and why the other is mgsinbeta and not the other way around

could someone explain to me why it is mgcosbeta

http://img213.imageshack.us/img213/8798/91559741.png [Broken]

Last edited by a moderator: May 6, 2017
2. Mar 8, 2013

### Staff: Mentor

Visualize what it looks like when the angle is set to 0. One will be 0 and the other will be mg. The one that is 0 is sin and the one that is mg is cos. That will always work.

Last edited: Mar 9, 2013
3. Mar 8, 2013

### rcgldr

It helps to understand this if you construct a triangle with mg as the hypotenuse, then it becomes clear when to use sin or cosine.

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Last edited: Mar 8, 2013
4. Mar 8, 2013

### physicsnoob145

thank you it is a little clearer now

5. Mar 9, 2013

### Johnahh

Just as rcgldr has shown the weight must always be the hypotenuse as it is largest force acting.