Can someone explain this to me.

[gvcalamike]

When your dealing with the x-component in projectile motion you work with the cosine of some angle. Yet, when your working with the x-component in force you work with the sin of some angle on inclined plane. It may seem like a silly question, but it keeps confusing me on my homework problems.

 

[mathman]

All that is involved is which angles are being referred to. It has nothing to do with the physics itself, only geometry.

 

[zgozvrm]

With projectile problems, you are generally dealing with a slope that is referenced to the ground, which is horizontal.

With force problems, you are generally dealing with a force that is offset from (referenced to) a gravitational force, which is typically vertical.

Look at the right triangles in each problem and see where the right angle is located. That is what determines whether you use sine or cosine.

 

[pergradus]

I never liked this convention. I always just changed my orientation to always reference all angles from the horizontal.

 

[AlephZero]

It may be confusing right now, but you really need to get used to handling this sort of thing. If you don't, it will keep confusing you in other topics that you haven't studied yet.

Make sure you are REALLY familiar with "sin = opposite / hypotenuse" etc, and practice identifying sin cos and tan in triangles with ANY orientation in a diagram.

 

[m.e.t.a.]

When I want the cosine or the sine of some angle between vectors, but I don't want to spend time chasing down which one I need, I just ask myself, "If the angle is small, do I expect the force (or velocity, flux, torque etc.) component to be large or small?". Large \Rightarrow cosine; small \Rightarrow sine. Hope this helps.

 

[tiny-tim]

When your dealing with the x-component in projectile motion you work with the cosine of some angle. Yet, when your working with the x-component in force you work with the sin of some angle on inclined plane. It may seem like a silly question, but it keeps confusing me on my homework problems.

hi gvcalamike!

it's always cos of the correct angle …

but sometimes the correct angle is 90° minus the one marked on the diagram!

however, personally i always use exactly the method that m.e.t.a. mentions!​