how is the second last force mentioned used?
Along with a pure torque it is used to replace a system of forces acting on a rigid body.
Net force
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
define unit of force using second law of motion UNANSWERABLE how does one define force? It is any interaction that, when unopposed, will change the motion of an object. who was the first to define it? The models regarding this were proposed by early thinkers which were corrected by Galileo Galilei, Sir Isaac Newton, and Einstein. where is the first person from? Pisa (then part of the Duchy of Florence), Italy. what is his profession? An astronomer, physicist and engineer. who was he married to? UNANSWERABLE when did he start his career as a scientist? After accidentally attending a lecture on geometry. what did the first law state about the former reaction? Newton's First Law of Motion states that objects continue to move in a state of constant velocity unless acted upon by an external net force (resultant force).
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Net force [SEP] Usage
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.