Editing Net force (section)

== Usage ==
[[File:Non-parallel net force.svg|thumb|279px|Vector diagram for addition of non-parallel forces.]]

In general, a system of forces acting on a rigid body can always be replaced by one force plus one pure (see previous section) torque. The force is the net force, but to calculate the additional torque, the net force must be assigned the line of action. The line of action can be selected arbitrarily, but the additional pure torque depends on this choice. In a special case, it is possible to find such line of action that this additional torque is zero.

The [[resultant force]] and torque can be determined for any configuration of forces.  However, an interesting special case is a torque-free resultant. This is useful, both conceptually and practically, because the body moves without rotating as if it was a particle.
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When the system of forces can be replaced by a resultant force, this can simplify practical calculations (e.g. in many planar systems, or using the center of gravity in homogenous field, etc.). On the conceptual level, definition of the resultant force underlines the fact that the net force does not fully replace the system of forces (so, for example, the [[work physics|work]] of the net force cannot replace the net work in the case of an extended rigid body, e.g. in the work-energy theorem etc.). And the concept is also useful for a full understanding of a more general approach.

[[File:Parallel net force.svg|thumb|185px|Vector diagram for addition of parallel forces.]]
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Some authors do not distinguish the resultant force from the net force and use the terms as [[synonym]]s.<ref>Resnick, Robert and Halliday, David (1966), Physics, (Vol I and II, Combined edition), Wiley International Edition, Library of Congress Catalog Card No. 66-11527</ref>