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Homework Help: Force on a Box - Find Weight

  1. Mar 8, 2010 #1
    1. The problem statement, all variables and given/known data

    You push down on a box at an angle 15° below the horizontal with a force of 650 N. If the box is on a horizontal surface and the coefficient of static friction is 0.7, what is the heaviest box you will be able to move?

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    So I've solved this problem using Fnet which was 650cos15 for the horizontal force and then subtracted [tex](\mu_s)(9.80)(m)[/tex] The ma part goes to zero since it's not accelerating.

    Therefore I get [tex]650cos15 = (\mu)(9.80)(m)[/tex] and then solve for m.

    I thought that was it. But now that I thinking about it more, there is a vertical component for the force. I'd imagine that extra force would make the box seem to weigh more, thereby increasing friction, making the box weigh less.

    Do I factor that vertical force in? If so, how?
     
  2. jcsd
  3. Mar 8, 2010 #2
    Ah! Maybe like this?

    [tex]650cos15 = [((\mu)(9.80)(m))+(650sin15)[/tex]

    All I did here was add in the vertical force component to the friction.

    Yeah...that isn't right since my box ends up weighing 2kg.
     
    Last edited: Mar 8, 2010
  4. Mar 8, 2010 #3
    Your above equation is correct.Though you will multiply the coefficient of friction to both the weight as well as 650sin15
     
  5. Mar 8, 2010 #4
    Nice.

    So something like this?


    [tex]650cos15 = [((\mu)(9.80)(m))+((\mu)(650sin15))][/tex]
     
  6. Mar 8, 2010 #5
    Yeah,that's correct.
     
  7. Mar 8, 2010 #6
    Thanks for the help!:approve:
     
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