1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Static Equilibrium Problem

  1. Sep 17, 2006 #1
    Here is the problem I am attempting to solve:
    http://img227.imageshack.us/img227/2747/problempr4.jpg [Broken]

    The large quadriceps muscle in the upper leg terminates at its lower end in a tendon attached to the upper end of the tibia. The forces on the lower leg when the leg is extended are modelled as shown where T is the tension in the tendon, C is the force of gravity acting on the lower leg, and F is the force of gravity acting on the foot. Find T when the tendon is at an angle of 25.0° with the tibia, assuming that C = 30.0 N, F = 12.5 N, and the leg is extended at an angle of 40.0° with the vertical. Assume that the centre of gravity of the lower leg is at its centre and that the tendon attaches the lower leg at a point one-fifth of the way down the leg.

    What I am uncertain is my method of solving this question. I'm quite sure that I'm missing something. Here is what I did:
    Fnet = 0
    Angle between quadricep and the dotted line is equal to 40.0°. Therefore, the angle between the quadriceps and T is equal to 65.0°.
    0 = Tsin65.0° - C - F
    T = 46.9 N

    Merci d'avance!
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Sep 17, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    HINT: The total moments of force about any point must be zero.
  4. Sep 17, 2006 #3


    User Avatar
    Homework Helper

    This is not correct, since you could write a scalar equation for the 'x-direction', too, which would be T*cos65 = 0 => T = 0. If you try to sum up the vectors graphically, you'll easily see that there's no state of equilibrium for this set of vectors.
  5. Sep 17, 2006 #4
    If they aren't in the state of equilibrium, does that mean the leg is moving down? Or are there reaction forces that help keep the equilibrium?

    *sighs* Torques...
  6. Sep 17, 2006 #5


    User Avatar
    Homework Helper

    To be honest, the medical context is killing me. :biggrin:
  7. Sep 17, 2006 #6
    Hmm .. as I'm thinking about this, there would have to be reaction forces to balance all the vectors. But where would they be situated? At the tendon? =S
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook