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Homework Help: Calculating force of the femur on the knee cap.

  1. Oct 3, 2012 #1
    1. The problem statement, all variables and given/known data
    The diagram of the leg shows the femur (1) and tibia (2). The quadriceps muscle (3) applies a force to the lower leg via a tendon (4) that is embedded with the kneecap (5). If the force applied by the muscle to the tendon is FM = 420 N, what is the force of the femur on the kneecap? A simplified model of the leg is shown next to the diagram. The leg bones are represented by two beams attached by a pin. The tendon is modelled by a rope and the kneecap acts like a pulley. The tendon above the kneecap makes an angle θ1 = 23° with respect to the vertical, and the portion of the tendon below the kneecap makes an angle of θ2 = 24° with respect to the vertical. Enter the x component, followed by the y component.


    2. Relevant equations


    3. The attempt at a solution

    I tried:

    Fx= max
    T1x + T2x + Ffemurx=0
    -240sin23 -240sin24 + Ffemur= 0
    -93.78 - 97.62 + Ffemur=0
    Ffemur= 191.4N

    T1y + T2y + Ffemury=0
    240cos23 - 240cos24 + Ffemury=0
    220.92 - 219.25 + Ffemury=0
    Ffemury= -1.67N

    I didn't get the right answer..

    please help!
  2. jcsd
  3. Oct 4, 2012 #2


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    Homework Helper

    Supposed that the tension is the same all along the tendon, and x means horizontal , y means vertical component of force, your solution looks correct. Try to use less significant digits.

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