## forceThe cause of a change in motion. Force is defined by Newton's second law as the rate of change of momentum or, equivalently, as the product of mass and acceleration ( F = m
× a). It is a vector quantity,
in the direction of the acceleration it brings about. Forces always occur
in equal and opposite action-reaction pairs between bodies, though it is
often useful to think of one body being in a force field. The SI (international system) unit of force is the newton. For example, if a 5-kg block is sitting at rest and is then pushed horizontally with an initial acceleration of 3 m/s ^{2}, the force that sets the
block in motion is 15 newtons (5 kg × 3 m/s^{2}). ## Related category• CLASSICAL MECHANICS | |||||

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