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Homework Help: Force on box

  1. Oct 1, 2007 #1
    1. The problem statement, all variables and given/known data

    (a) A box is placed on an frictionless inclined plane with an angle of 18° from the horizontal. The box has a mass of 35 kg, what is the magnitude of the acceleration of the box along the inclined plane?
    (b) What magnitude of force would you need to apply to the box in order to keep it at rest? Assume there is no friction and the force applied is parallel to the ground.

    2. Relevant equations


    3. The attempt at a solution

    using a combination of the two equations I got that the answer for the first part was 3.03 m/s2 (which was right), but I'm still rather confused about the second part. Do I have to set a=0, because that would mean that F would have to be zero (which wouldn't make sense....and isn't right either). I guess I mainly need help in setting this problem up and the reasoning behind it.
  2. jcsd
  3. Oct 1, 2007 #2


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    Yes, it does mean you have to apply another force parallel to the incline to have [tex] \sum F = 0 [/tex]. Once the equilibrium position is reached the box moves with constant velocity, in this case it would be zero (i.e., at rest).
  4. Oct 1, 2007 #3
    Think about what forces you would need to act against in order to keep the box at rest.

    Now that you've thought about that, think about what force YOU would need to apply in order to keep the box at rest.

    Hint: We know if the box is at rest, then Fnet = 0.

    Let me know if this helps. If it does not, I can make it a bit more clear.
  5. Oct 1, 2007 #4
    well, since it is a frictionless surface, you would need to conteract against gravity....and if Fnet=0....the only way you could get that would be if you subtracted the original force so....do you basically have to use the acceleration from part a to solve for F and that would be your answer? (or am I off track....)
  6. Oct 1, 2007 #5
    also, because it says that the force is going to be parallel to the ground, do I need to include the sin of the angle, or would that already be incorporated into my answer?
  7. Oct 2, 2007 #6
    ummm....I'm still not really sure if this is right....
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