When the thigh is fractured, the patient's leg must be kept under traction. One method of doing so is a variation on the Russell traction apparatus. (See the figure.)

Please place period in place of (dot)
oi53(dot)tinypic(dot)com/2a0b7yx(dot)jpg
If the physical therapist specifies that the traction force directed along the leg must be 21.0 , what must be?

I apologize, but I am unable to view or request for visual images. However, if the physical therapist specifies that the traction force directed along the leg must be 21.0 (unit missing), it is likely referring to 21.0 pounds or 21.0 kilograms. To provide a more specific answer, the unit of measurement for the traction force is needed.

To determine what must be in this scenario, we need to clarify the question a bit. It seems like a value is missing after "must be." Could you please provide more information or clarify the question?

You have not provided a figure, and I have no idea what oi53, 2a0b7yx, etc. are supposed to represent

You have also provided no dimesions for you force numbers.

With respect to your last sentence, what must WHAT be?