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Can someone please help me with this physic problem?

Discussion in 'Pre-Medical - MD' started by NubianPrincess, Jun 16, 2002.

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  1. NubianPrincess

    NubianPrincess Perpetually Bored

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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?????
     
    #1 NubianPrincess, Jun 16, 2002
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  2. Legend

    Legend Super Senior Member

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    Look at the 'triangle' in the diagram.

    The component parallel to the inclined plane is the force you need to calculate friction coefficient.
     
    #2 Legend, Jun 16, 2002
  3. moo

    moo 1K Member

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    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.
     
    #3 moo, Jun 16, 2002
  4. Legend

    Legend Super Senior Member

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    </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by NubianPrincess:
    but I thought that the x componet would involve cos, not sin?????[/QB]</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">If you rotate 'x-component' about 90 degrees, it becomes 'y-component'.

    Look at the triangle carefully. That triangle seems somewhat inverted, right?
     
    #4 Legend, Jun 16, 2002
  5. Flack Pinku

    Flack Pinku U lookin at my glasses??

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    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. :)
     
    #5 Flack Pinku, Jun 16, 2002
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