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Homework Help: Mechanics Help

  1. Apr 19, 2008 #1
    Hi,

    I'm not getting some mechanics questions. I'm sure that I'm right... but the answer in the back of the book is different. I'll type it out for you guys

    First Question:

    "A body of mass 2kg is held in limiting equilibrium on a rough plane inclined at 20 degrees to the horizontal by a horizontal force X. The coefficient of friction between the body and the plane is 0.2. Modelling the body as a particle find X when the body is on the point of slipping:

    a) up the plane.
    b) down the plane."

    I done a, but got it wrong, so I left out b, since I'm assuming I'm missing something.

    I got 11.05 N, but the answer says 11.9N

    The Second question is:

    "Given that the resistances total 400N find the magnitude of the constant force needed to accelerate a car of mass 800kg from rest to 20m/s in 15 seconds."

    Okay so I work out a, which is a=(v-u)/t=20/15=4/3

    So F=ma=800x4/3=1066.67N

    F+Resistance=Total Force=1466.77N, but the answer is 1470N apparently?? I'm confused.

    Would like this to be resolved as soon as possible cos I've got alot of mechanics questions to do.

    Thanks :)
     
  2. jcsd
  3. Apr 19, 2008 #2

    Doc Al

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    Staff: Mentor

    For the first problem: Show your work not just your answer.

    For the second: They just rounded off to a reasonable number of significant figures; your answer matches.
     
  4. Apr 19, 2008 #3
    Okay gonna be a bit difficult as you need a force diagram. But just imagine a box on an inclined plane 20 degrees to the horizontal, with mg acting down, Normal Reaction and Frictional Force.

    Normal Reaction=19.6Cos20=18.4N

    Therefore Frictional Force=18.4x0.2=3.68N

    So Xcos20=3.68+19.6sin20

    X=11.05N

    And the answer in the back of the book is 11.9N??
     
  5. Apr 19, 2008 #4

    Doc Al

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    Staff: Mentor

    Careful. Since the applied force is horizontal, it affects the normal force.
     
  6. Apr 19, 2008 #5
    Okay... how do I take that into account?? I don't know X in the first place? Give me a hint please :)
     
  7. Apr 19, 2008 #6

    Doc Al

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    Staff: Mentor

    Find the normal force (in terms of X) by analyzing the force components perpendicular to the incline surface.
     
  8. Apr 25, 2008 #7
    Thanks for your help.
     
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