# Finding the force of the femur on the knee cap.

1. Oct 4, 2012

### meaganl

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.

https://www.physicsforums.com/attachment.php?attachmentid=51494&thumb=1&d=1349314642

2. Relevant equations

fnet=ma

3. The attempt at a solution

I tried:

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

Fnety=may
T1y + T2y + Ffemury=0
240cos23 -240cos24 + Ffemury=0
220.92-219.25 + Ffemury=0
Ffemury=-1.67N
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 4, 2012

### ehild

I see only very small picture. Can not you show a bigger one?

ehild

3. Oct 4, 2012