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Homework Help: Newtons laws of motion, tension problem

  1. Feb 25, 2009 #1
    1. The problem statement, all variables and given/known data
    Due to a jaw injury, a patient must wear a strap (see the figure) that produces a net upward force of 5.00 N on his chin. The tension is the same throughout the strap.


    To what tension must the strap be adjusted to provide the necessary upward force?

    2. Relevant equations

    3. The attempt at a solution
    I already know the answer its 3.15 N I cant figure out how to get it though. I'm assuming its a equilibrium problem and if it needs an upward force of 5N the force due to gravity would also be 5, right?
  2. jcsd
  3. Feb 25, 2009 #2
    Use vectors and trig. The net upward forces of the two straps must equal 5 newtons. Draw a vector diagram with two right triangles.

    You first need to solve for the horizontal forces. Then use trig to find the hypotenuse of your right triangle. Divide that answer by two (the force needs to be distributed equally between the straps), and you get 3.15N.
  4. Feb 25, 2009 #3
    Thanks for the response! Im not sure where to start with finding the horizontal forces. The y vector component 5sin(angle)?? When I make the two right triangles, I cut the 75 in half correct?
  5. Feb 26, 2009 #4
    How do I know the force due to gravity is actually 5n? I plug it in to check my answer and I keep getting something other then 0. How can I calculate the force due to gravity? What about normal force?
  6. Feb 26, 2009 #5


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    Homework Helper

    that produces a net upward force of 5.00 N on his chin. The tension is the same throughout the strap.
    The above statement takes into account all the forces including the gravitational force.
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