Interaction of bodies. Force

When bodies act on each other, their velocity or shape changes. The action between bodies is always mutual: when we kick a ball, the ball also acts on the foot. The vector quantity measuring this action is force, denoted F. A force is determined by its magnitude, direction and point of application. The SI unit of force is the newton (N); kilonewtons and millinewtons are also used: 1 kN = 1000 N, 1 N = 1000 mN. Force is measured with a dynamometer. When several forces act, their vector sum is the resultant force: forces in the same direction add, opposite forces subtract.