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Homework Help: Calculate the moment mechanics problem

  1. Oct 12, 2007 #1
    1. The problem statement, all variables and given/known data

    Calculate the moment of the 90N force about point O for the condition theta=15 degrees.

    2. Relevant equations

    Moment= Force* perpendicular distance

    3. The attempt at a solution

    I am doing the first step

    Fx= 90sin theta
    Fy= 90costheta

    What i got was Moment= (Fx*0.8) + (Fy*0.6)= 72sintheta + 54costhetha

    When I looked at the worked solution, they got a -Fx*0.8, i dont understand how the negative sign came about. Can you help out? Thanks

    Attached Files:

  2. jcsd
  3. Oct 13, 2007 #2


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    Staff Emeritus
    Science Advisor

    Clockwise moments are consider +, counterclockwise moments are considered -. Righthand rule.
  4. Oct 18, 2007 #3


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    Science Advisor

    You need to look at your units as well. As an early student, write your units out for everything. Degrees/radians are unitless. All you have right now is a force times a unitless quantity, your resultant is N. You need Nm, which means you need to still multiply by a distance.

    You were correct to take the sin/cos to find the horizontal and vertical components of the force, but you still need to multiply it by the distance from the center.

    So, for the vertical direction:

    M = -90N*cos(theta)*600mm*(1 m / 1000 mm) = x Nm

    As Astro said, negative because of sign convention. Do the same for the other component, making sure its in the direction that you think.
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