Can someone please help me with this physic problem?

[NubianPrincess]

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This is the question:
A box slides down a 30 degree ramp with an acceleration of 1.20m/s^2. Determine the co-efficient of kinetic friction between the box and the ramp.
The solution to this problem is <a href="http://physics.baruch.cuny.edu/summer2k2/sol0403.gif" target="_blank">here</a>.
I understand the theories involved, but my problem is I am unclear how to determine the components of the forces when the object is on an incline. I dont understand how to know that the component in the x direction is = Fg*sin30.
Please help me if you can, please? 🙂I understand where im getting stuck - the method seems to be to break the force of gravity down into x and y components, but I thought that the x componet would involve cos, not sin?????

 

[Legend]

Look at the 'triangle' in the diagram.

The component parallel to the inclined plane is the force you need to calculate friction coefficient.

 

[moo]

You can reason this out from simple geometry (similar triangles).

Sorry I can't be of more help but it's really hard to teach someone this stuff w/o drawing the diagrams.

 

[Legend]

•••quote:•••Originally posted by NubianPrincess:
but I thought that the x componet would involve cos, not sin?????[/QB]••••If you rotate 'x-component' about 90 degrees, it becomes 'y-component'.

Look at the triangle carefully. That triangle seems somewhat inverted, right?

 

[Flack Pinku]

In my opinion, if you reason these things out, you'll do much better on any test if you know the "why's"... with that said:

Look at the angle in the diagram (30 degrees). cos = adjacent/hypotoneuse. If you want the adjacent side to that 30deg, you have the "y" component of Fg (cos(30) goes vertically down in this case, NOT sin(30)). So, Fg*cos(30) is the Normal force (force perpendicular to the plane).

sin = opposite/hyp. So following the logic given before, Fg*sin(30) is the "radial" (or the "x") component down-and-along the plane.

You know that Fg*sin(30) - mu*Fg*cos(30) = ma.

Let me know if this helps or not. I hope I helped. 🙂