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Homework Help: Force needed to keep object from accelerating problem

  1. Oct 7, 2004 #1
    This problem is keeping me up all night, I did what I thought was right to figure out the force needed but the answer sheet tells me that my solution is wrong, spend 4 hours trying to figure out what I did wrong can't come up with anything:

    A 280kg box slides 4.3m down a 30 degree incline and is kept from accelerating by a man who is pushing back on it parallel to the incline. The effective coefficient of kinetic friction is 0.4. Calculate the force exerted by the man.

    It might well be that I am completely out of my mind today and forgetting some basic thing that I am supposed to do, anyways I tried to do:

    Ff = 280 * 9.8 * sin30 * 0.4

    I am quite sure actually that I am forgetting something..... just god damn don't know what it is.

    thnx for any help in advance.
  2. jcsd
  3. Oct 7, 2004 #2
    Try [tex]\sum F=0[/tex]

    Now, as for the forces you seem to be missing one. Draw a FBD. Put ALL forces acting on the box. Sum them in the x and y directions. If you need help determining the forces post what you've got thus far and we'll see if we can figure out which force(s) is/are missing.

    Good luck

    PS welcome.
  4. Oct 7, 2004 #3
    hrm well the FBD that I drew I set up the x axis to be parallel to the incline and basically I have the following forces:

    g, pulling the object down (is split into two forces since it is @ an incline the force pulling the box down the incline should be sin30 of the full force, I believe)
    coefficiant of friction force (0.4) going against g
    force that the person excerts on the box also going against g
    and normal force which I believe should be equal to cos60 of the g force
  5. Oct 7, 2004 #4
    So you have:

    [tex]\sum F_x=W_x-F_{fr}-F_{man}=0[/tex]

    You know how to find the component of the weight vector in the x direction. That Wieght of the box is pulling the box to the right (positive). The man and friction are pushing to the left (negative).

    How do you determine the force of friction? Once you figure this out you'll have all the parts needed to solve this question.
  6. Oct 8, 2004 #5
    that is where I get lost I mean okay I determine the force of friction which should be equal to:
    coefficient of friction * netForce

    now net force will be mass * acceleration (all in x-axis)

    this I can do with: mass * acceleration * sin30

    and so here I get lost as to what I should do next


    never mind I got it, seems I was messing up one cos angle and that was causing all the problems :grumpy: duh :yuck: god I'm stupid :tongue:
    Last edited: Oct 8, 2004
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