1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Science Advisor
    Homework Helper

    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

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

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

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook