Forces are not on an object, but between objects (that is the gist of Newton's third law).

If an object is not accelerating, then the forces acting on that object must add up to zero. If the object is interacting with two other objects then the forces for those interactions must be equal and opposite.

If you have three scales conected in series, with people pulling at the two end scales, you can look at all of the forces. You have person A pulling on scale 1 with a force of 10 Newtons. By Newton's third law scale 1 is then pulling person A with a force of 10N. Scale 1 connects only with person A and scale 2 (the middle scale). By equilibrium the force between scale 1 and scale 2 must be 10N. Scale 2 connects only with scale 1 and scale 3. Since it is in equilibrium (not accelerating) the force between scale 2 and scale 3 must be the same as the force between scale 2 and scale 1, i.e., 10N (but directed in opposite directions, as seen from scale 2). Scale 3 connects to scale 2 and person B. Again, by equilibrium, the force between scale 3 and person B must again be 10N.

Note that part of the description is redundant. If you have person A pulling at one end of an object (whether it be a simple rope or a chain of objects, such as scales) and person B pulling at the other end, if the object is not accelerating the force exerted by person A must be the same as the force exerted by person B.

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� � � � Tom Gutman