Forces and Motion

The history of the force concept

Stories from Physics for 11-14 14-16 IOP RESOURCES

The modern concept of force has progressed through a number of iterations. Aristotle argued that ‘everything moved must be moved by something’ and distinguished between forces inherent in objects and forces that emanate from substances. Writers in the 14th-16th centuries promoted the Aristotelian concept of motive force and, despite Galileo’s critique of the idea in the Dialogues Concerning Two New Sciences, the concept was still being used by members of the Royal Society as late as the 1690s. One of the proponents of the Aristotelian model, John Buridan, argued that a permanent force was impressed on a projectile unless resistive forces acted and that the force was proportional to the quantity of matter and speed of the object. The assertion that ‘whenever A acts upon B, B reacts upon A’ was mentioned as early as 1564 by Valles and was widely held by physicists from the 1630s onwards.

Like Aristotle, Newton in the Principia, refers to two kinds of forces: Vis insita, inertial forces which are seen as inherent to bodies and vis impressa, forces exerted on a body, such as pressure and impact forces. Newton argued that vis insita was proportional to the mass of the body. Newton tended to work through the use of proportions rather than the use of equations: the familiar formula giving the magnitude of gravitational attraction between two masses is not found in the Principia. Leibnitz proposed an alternative classification of forces, arguing that moving force (vis mortix) came in two forms: living force (vis viva) and dead force (vis mortix). Dead force was conceptualised as having a drive to motion without an accompanying motion, for example, the force in a stretched spring.



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