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Vector Question Assistance

  1. Feb 26, 2009 #1
    1. The problem statement, all variables and given/known data
    Three Forces are applied to a large boulder in an attempt to move it. The Forces are F1=(600N, 130 degrees), F2=(780N, 200 degrees), and F3=(550N, 165 degrees). What is the net force acting on the boulder? If the boulder does not move, what is the resistive force acting

    2. Relevant equations
    Vx= Vcos
    Vy= Vsin

    3. The attempt at a solution
    Used Vx= Vcos and Vy= Vsin for each force and got

    y: 450+(-267)+(142) = 325
    x: (-386)+(-705)+(-531) = -1622

    1622(-i) + 325 (j) for the resulatant or net force.

    the second part of the question is what im stuck on. i guess from the resultant the boulder is moving so there has to be a resistive force but i don't know how to get there.
  2. jcsd
  3. Feb 26, 2009 #2


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    Homework Helper

    It seems part of your question is missing. But in finding your net force from the 3 forces, it doesn't require that the rock moves, if that is your thinking.

    There's that action reaction thing to keep in mind and knowing their net force doesn't mean that the rock necessarily moves. Press down on your desk. Let's say it doesn't move, but in not moving it doesn't mean that you applied no force. In not moving it merely indicates that the resistive force was up to the task of not allowing the object to move. If the net of the 3 forces and the resistive force is positive, then it will begin to move by F = m*a.
  4. Feb 26, 2009 #3
    i don't think i'm missing anything from the question cause thats all it says for me. i did the net force part right though?
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