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I cant figure out these questions.HELP

  1. Nov 30, 2003 #1
    I cant figure out these questions.......HELP!!!!

    1. A horizontal force of 150N is used to push a 40kg bag a distance of 6.00m on a rough horizontal surface. If the crate moves with constant velocity:
    a. what is the work done by the force
    b. what is the coefficient of kinetic friction

    How do i start this???
  2. jcsd
  3. Nov 30, 2003 #2
    Just a repeat answer since the symbols didn't come out correctly ...

    The Work Done = Change in Kinetic Energy, which means that:

    [tex]F * d = m/2(v_{f}^2 - v_{i}^2)[/tex],where,
    [tex]v_{i}[/tex] & [tex]v_{f}[/tex] are the initial and final velocities respectively and [tex]F[/tex] is the force and [tex]d[/tex] is the displacement. In your case the initial velocity is zero, hence the work done is directly equal to the kinetic energy.

    From this you can find the velocity of the object.

    As for your question the work done is directly the Force * distance that the force has pushed the object to.

    Kinetic Friction is basically the friction that is exerted by the surface on a body that is moving. Thus the Kinetic Frictional force = [tex]F = \mu_{k}*mg [/tex], where [tex]\mu_{k}[/tex] is the coefficient of kinetic friction and [tex]F[/tex] is the force applied.This is done as the frictional force always acts in a way to oppose motion. Hence the magnitude of the frictional force = magnitude of the applied force.

  4. Nov 30, 2003 #3
    hey sridhar what is [tex]?
    thanks for the help
  5. Nov 30, 2003 #4
    tex is basically the latex tag that is used to begin typing latex commands that can be used to generate symbols like I have in my 2nd answer. I screwed up on the tags in the previous answer, so I have corrected them in my second Just click on one of those tags to see how the symbols are got(Those Huge black formulae...jus click on one of them...go ahead)

    u can use latex here to type something like this:

    [tex]\sin^2\Phi+\cos^2\Phi = 1[/tex]

    Just click on the Huge Black Formula above to see the Latex code.

  6. Nov 30, 2003 #5
    You can also do something like this using Latex:

    [tex]\int_{0}^\infty f(x)\partial x[/tex]

    or something like:

    [tex]\sum (n) = n(n+1)/2[/tex]

    So as you see the display beomes more organised wit latex. Yet the time taken to type an answer using latex is enormous..

  7. Nov 30, 2003 #6


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    Staff Emeritus
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    Gold Member

    A hint for you, there is a edit button at the bottom of each message, if you wrote it, you can edit it! :smile:
  8. Nov 30, 2003 #7

    Doc Al

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    Staff: Mentor

    Re: I cant figure out these questions.......HELP!!!!

    For part a, consider the definition of work.
    For part b, consider what the force of friction must equal since the crate has a constant velocity.
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