A Hospital patient has injured her leg, how much mass must be used to keep her leg stationary.  The leg has a mass of 2.6 kg and is 0.87 meters in length, the rope for the tension mass  is connected at 0.65 meters from the knee.

1 answer

well force up = force down
force up at knee = Fk at x = 0
Force up on rope = T at x = .65
weight down at center = 2.6*9.81 at x = .435

forces up = weight down
Fk + T = 25.5 N
moments about knee = 0
0 = .65T - 25.5*.435
so T = 17.1 Newtons
if this is done by a mass on a pulley the mass = 17.1/9.81 = 1.74 kg