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Homework Help: Easy Homework Problem

  1. Feb 21, 2010 #1
    1. Figure 7-30 shows an overhead view of three horizontal forces acting on a cargo canister that was initially stationary but that now moves across a frictionless floor. The force magnitudes are F1 = 2.28 N, F2 = 4.54 N, and F3 = 8.24 N, and the indicated angles are θ2 = 51.6° and θ3 = 35.8°. What is the net work done on the canister by the three forces during the first 2.70 m of displacement?



    2. W=Fdcostheta
    Wnet=W1+W2+w3




    3. I really thought this was an extremely easy problem to do but apparently not. Here's my attempt. Could someone please tell me what I did wrong?

    W1=(2.28)(2.70)cos0=-6.156J
    W2=(4.54)(2.70)cos51.6=-7.614J
    W3=(8.24)(2.70)cos35.8=18.045J
    Wnet=(18.045)+(-6.156)+(-7.614)= 4.28J
     
  2. jcsd
  3. Feb 21, 2010 #2

    Andrew Mason

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    Perhaps you could post or at least explain the diagram. It is a bit easier to find the resultant (net) force: [itex]\sum \vec F_i = \vec F_{net}[/itex] and then use:

    [tex]W = \vec F_{net} \cdot \vec d[/tex]

    AM
     
    Last edited: Feb 21, 2010
  4. Feb 21, 2010 #3
    Here's a diagram: http://imgur.com/DAJ8G.gif

    So would I find the magnitude of the components add them up then multiply that by the distance?
     
  5. Feb 21, 2010 #4

    Redbelly98

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    Well, you would find the magnitude of the net force by adding up the force components. Then use work = force*distance.

    Your earlier attempt (shown in post #1) assumed the object moved in the +x direction, which isn't necessarily true.
     
  6. Feb 21, 2010 #5
    so i would find the y and x components for each of the forces then add them up and use Pythagorean theorem then multiply that by the distance?

    For the angle of F2 should it be 270-51.6=218.4 degrees and should angle of F1 be 180 degrees?
     
  7. Feb 21, 2010 #6

    Redbelly98

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    Yes to all statements.
     
  8. Feb 21, 2010 #7
    I got 5.86 J does that sound right?
     
  9. Feb 21, 2010 #8

    Redbelly98

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    I don't know, I haven't worked through the problem myself and you didn't show your work. So I really can't say at this point ... but you could remedy the situation :wink:
     
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