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Homework Help: 2 Questions - Fnet with a Vector + Forces

  1. Feb 18, 2009 #1
    :confused: :confused: :confused:

    First question

    Harry and Ashley were pulling a sled. Harry pulled at 15 degrees West of South at 400N and Ashley pulled at 50 degrees North of West at 375N. They encountered friction to the east at 150N. Calculate the overall force (Fnet) on the sled, using a scaled vector drawing. Use a scale of 50N to 1 cm.

    I have the drawing finished, but I don't know how to figure out Fnet? I am really confused...

    Second Question

    A rightward force is applied to a 10-kg object to move it across a rough surface at constant velocity. The coefficient of friction, µ, between the object and the surface is 0.2. Draw a free body diagram to determine the graviational force, normal force, applied force, frictional force, and net force. Neglect air resistance.

    For this one, I have the diagram, basically a square with an arrow from each Side

    Upwards is Fnorm, Right is Fapp, Downwards is Fgrav, and Left is Ffric.

    I have no idea how to do this one either. I'd appreciate some help, thanks. :redface:
    Last edited: Feb 18, 2009
  2. jcsd
  3. Feb 18, 2009 #2
    For your first question, the easiest solution is to break all the forces up into there x and y components and then solve them this way

    for example, the force that Harry exerts could be broken into
    [tex]F_{x}= -400N sin(15)[/tex]
    [tex]F_{y}= -400N cos(15)[/tex]
    Once you do this for all the forces you can add them up easily (it may be easier to draw the forces again once you have broken them up)

    For your second question it takes more thought and less calculation
    Think about how the Normal force relates to the gravitational force
    Also think about the relationship between the frictional force, and the force being exerted on the object

    If that still does not make it clear, think about a block on a table. What happens when you push it. What does that tell you about the net force acting on it. Remember that you diagram must represent what actual happens in the physical case.
  4. Feb 18, 2009 #3
    Sorry, but I still do not understand. We haven't used sin or cos in any of our equations yet so I do not see how I use that..?

    And for your explanation on my second question, I don't understand that either sorry.

    This class is basically an introductory course for people who have never ever taken physics before, so you guys will have to explain how to do the questions to me in laymans terms. :P


    I figured out the first question, I think... following your help. I got 38.46 after adding up both the Fx and Fy numbers for Harry and Ashley then subtracting them. Thanks!
    Last edited: Feb 18, 2009
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