The device shown in the figure below (Figure 1) is one version of a Russell trac
ID: 1787898 • Letter: T
Question
The device shown in the figure below (Figure 1) is one version of a Russell traction apparatus. It has two functions: to support the injured leg horizontally and at the same time provide a horizontal traction force on it. This can be done by adjusting the weight W and the angle . For this patient, his leg (including his foot) is 85.0 cm long (measured from the hip joint) and has a mass of 15.6 kg . Its center of mass is 40.0 cm from the hip joint. A support strap is attached to the patient's ankle 13.0 cm from the bottom of his foot.
What weight W is needed to support the leg horizontally?
If the therapist specifies that the traction force must be 12.0 N horizontally, what must be the angle ?
What is the greatest traction force that this apparatus could supply to this patient's leg?
What is in that case?
Explanation / Answer
A) The torque about the hip must = 0, or the leg would continue to rotate; also, the tension in the strap is equal to the hanging weight. Then
W * (85 - 13)cm = 15.6kg * 9.8m/s² * 40cm
W = 84.93 N
B) The horizontal component of the strap tension on each side of the foot pulley must be 12 N; therefore the angle = arctan(70/12) = 80.27º
C) When = 0º, the tension is 2W 2*84.93 N = 169.86N
D) = 0º