Question

When you bend your knee, the quadriceps muscle is stretched. This increases the tension in the quadriceps tendon attached to your kneecap (patella), which, in turn, increases the tension in the patella tendon that attaches your kneecap to your lower leg bone (tibia). Simultaneously, the end of your upper leg bone (femur) pushes outward on the patella. The figure shows how these parts of a knee joint are arranged.(Figure 1)

What size force does the femur exerts on the kneecap if the tendons are oriented as in the figure and the tension in each tendon is 60 N?

 

Answer 1

Answer 2

net x = 60*cos(42)

net y = 60 - 60 *sin(42)

so net force = sqrt( (60*cos(42))^2 + (60-60*sin(42))^2)=48.8 N

Answer 3

?F(horiz) = 0
Fh = 60 cos 42 = 44.58

?F(vert) = 0,
Fv = 60 - 60 sin 42 = 60 - 40.148 = 19.85

F = ? Fh