Other meanings of Force
Physics
In physics, a force is an influence that can change the motion of an object. It is a vector quantity, possessing both magnitude and direction, and is measured in newtons (N). Forces arise from interactions between bodies, such as gravity, electromagnetism, and contact pushes or pulls. The concept is formalized by Newton's laws of motion, which describe how forces cause accelerations and how forces always occur in pairs.
A force is defined operationally as any influence that causes a massive body to accelerate. Newton's second law quantifies this: the net force on an object equals the time rate of change of its momentum, often written as F = ma for constant mass.1 The first law states that in the absence of a net force, a body remains at rest or in uniform motion, establishing inertia as a fundamental property. The third law asserts that forces always come in equal and opposite pairs, so that interactions are mutual.2 These laws, published in the Principia (1687), replaced earlier Aristotelian notions of motion and remain the cornerstone of classical mechanics.
Forces are broadly classified into contact forces and action-at-a-distance forces. Contact forces include friction, tension, normal force, and air resistance, which arise from microscopic electromagnetic interactions between surfaces. Non-contact forces include gravity, electromagnetic force, and the nuclear forces. In modern physics, all known forces are attributed to four fundamental interactions: gravitational, electromagnetic, strong nuclear, and weak nuclear.3 Everyday forces like pushes, pulls, and friction are manifestations of these fundamentals, with gravity dominating at macroscopic scales and the strong force binding atomic nuclei.
The SI unit of force is the newton (N), defined as the force required to accelerate a one-kilogram mass by one meter per second squared. Force is measured using devices such as spring scales and load cells, which rely on Hooke's law or strain gauges. In the CGS system, the unit is the dyne (1 N = 10⁵ dyn), and in imperial units, the pound-force is still used in some engineering contexts. Calibration of force sensors is traceable to national standards, ensuring consistency in industrial and scientific applications.
Beyond the basics, force has subtle and surprising facets. For instance, the concept of impulse (force integrated over time) is central to collision analysis, yet often overlooked. The normal force is not always equal to weight; on an incline or accelerating elevator, it adjusts to maintain perpendicular contact. Historically, the term "force" was used by medieval philosophers like Jean Buridan for impetus, a precursor to momentum.4 In relativity, force transforms between frames, and in quantum mechanics, forces are mediated by exchange particles (e.g., photons for electromagnetism). The strong force, which binds quarks, actually increases with distance, leading to quark confinement—a counterintuitive behavior unlike gravity or electromagnetism.
This article focuses on the classical and modern physics concept of force, distinct from other uses such as 'force' in law or military contexts.
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