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Homework Help: Mechanics vector problem

  1. Jun 30, 2016 #1
    <<Mentor note: Missing template due to being moved from other forum.>>

    I took this problem from Vector Mechanics for Engineers by Beer et al. The reason why I am writing this is because I don't understand where I am wrong in this problem, yet I know I am wrong.

    This is problem 6.123 and it's a problem concerning analysis of structures (in this case a machine).
    So the first thing I did was calculating the reactions the system has. In this case there is a reaction in node A which consists of a rough surface. Therefore there are two forces which is in fact one single force but decomposed into its horizontal and vertical components which I called Ax and Ay respectively.
    This is where my problem begins, lets suppose I just considered the reactions Ax and Ay in node A and the 250 N load acting in node C. Then I work with equilibrium equations:
    [itex]\sum F_{x}=0[/itex]
    [itex]\sum F_{y}=0[/itex]
    For the equation [itex]\sum F_{x}=0[/itex] we find out that the only force acting upon the x-axis is the horizontal component Ax that becomes zero. Then for the other equation [itex]\sum F_{y}=0[/itex] I see that there are two forces: the 250 N that goes downwards and the Ay that I supposed that goes upwards. When I perform the operations required I get that the Ay force is 250 N that goes upwards. Since the vertical component is the only component for the reaction in node A then I conclude that the reaction in node A is 250 N upwards.
    So far we have answered part b).
    When I consult the answers to selected problems section just to verify I am right I see that the answer doesn't match:
    It is not that the answer doesn't match in magnitude, but it also says that the reaction has an angle of 61.3°.
    I don't know where I am wrong.
    Last edited by a moderator: Jun 30, 2016
  2. jcsd
  3. Jun 30, 2016 #2


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    Science Advisor

    You've ignored what's going on at B (and/or D). They contribute forces in both x and y.

    Complete question a) before proceeding to b).
    Taking moments about A is a good first step.
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