Force & Evolution
In physics, a force is an influence that can cause an object to change its velocity, i.e., to accelerate, unless counterbalanced by other forces. The concept of force makes the everyday notion of pushing or pulling mathematically precise. Because the magnitude and direction of a force are both important, force is a vector quantity. It is measured in the SI unit of newton (N) and represented by the symbol F.
Force plays a central role in classical mechanics, figuring in all three of Newton's laws of motion.
Types of forces often encountered in classical mechanics include elasticity, friction, contact or "normal" forces, and gravity. The rotational analogue of force is torque, which produces changes in rotational speed of an object. In an extended body, each part usually applies forces on the adjacent parts; the distribution of such forces through the body is the internal mechanical stress. Such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate. Stress usually causes deformation of solid materials, or flow in fluids.
Special relativity, general relativity, and quantum mechanics revise the law and its meaning and modern physics focuses on fundamental interactions as the ultimate origin of force, but the understanding of force provided by classical mechanics remains entirely satisfactory for many practical purposes.
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