Incline plane problem

[drgshahbaz1193]

Hi,
I was working through the problem on page 21 in the physics section and I got really confused. For the X vector component sin was used and for the Y component cosine was used? I thought that sin was always used for the Y component? Help Please:?:

[mcat_premed3832]

Organic chemistry and physics are only fun if you limit memorization to the basic rules and then use those rules to problem solve. There is no rule regarding using a particular function for the x or y axis. But there are rules for functions (i.e. sine = opposite/hypotenuse, etc.; www.goldstandard-mcat.com/physics-equation-lists/).

Resolving into x and y components can often be done in more than just one way. The key is that, if events are occurring at various angles, involving vectors, that you identify a frame of reference and then stick to it. A common (but not exclusive) frame of reference for inclined problems is to take the axis of the surface of the inclined place as the x axis. It makes it easier to not have to resolve the acceleration "a", nor the friction "f", nor the force normal "N" into x and y components.

Once the decision has been made to make the incline of the plane the x axis, then you see from the diagram that the weight W must be directed straight down (really directed to the center of the Earth) and so it must be resolved into 2 smaller components (i.e. W must be the strongest of the forces - the hypotenuse - divided into 2 smaller components - x and y). Once the triangles are drawn with W as the hypotenuse, then we are only following the rules of functions.

Please note: if it was a classic projectile problem and you were trying to resolve the initial velocity, then, since sine = opp./hyp. then indeed the opposite is always the y component.

[DLBMCAT]

Great response, very helpful!