力 | 定義と公式 | ブリタニカ

Force | Definition & Formula | Britannica Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos force Introduction References & Edit History Related Topics Images & Videos Quizzes Physics and Natural Law Contents CITE verified Cite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style MLA APA Chicago Manual of Style Copy Citation Share Share Share to social media Facebook X URL https://www.britannica.com/science/force-physics Feedback External Websites Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback Our editors will review what you’ve submitted and determine whether to revise the article. External Websites California Institute of Technology - The Feynman Lectures on Physics - Characteristics of Force Oberlin College - What is force? What does force do? Open Library Publishing Platform - College Physics – Fundamentals and Applications - Concepts: Introduction to Forces; NewtonÂ’s Laws Physics LibreTexts - Force The Physics Classroom - The Meaning of Force OpenStax - Physics - Force Britannica Websites Articles from Britannica Encyclopedias for elementary and high school students. force - Children's Encyclopedia (Ages 8-11) force - Student Encyclopedia (Ages 11 and up) Two forces applied simultaneously to the same point have the same effect as a single equivalent force. The resultant force can be found by constructing a parallelogram with the initial force vectors forming two adjacent sides. The diagonal of the parallelogram gives the resultant force vector. (more) force physics Ask Anything Homework Help Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated May 2, 2026 • History Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything force , in mechanics , any action that tends to maintain or alter the motion of a body or to distort it. The concept of force is commonly explained in terms of Isaac Newton ’s three laws of motion set forth in his Principia Mathematica (1687). According to Newton’s first principle , a body that is at rest or moving at a uniform rate in a straight line will remain in that state until some force is applied to it. The second law says that when an external force acts on a body, it produces an acceleration (change in velocity) of the body in the direction of the force. The magnitude of the acceleration is directly proportional to the magnitude of the external force and inversely proportional to the quantity of matter in the body. Newton’s third law states that when one body exerts a force on another body, the second body exerts an equal force on the first body. This principle of action and reaction explains why a force tends to deform a body (i.e., change its shape) whether or not it causes the body to move. The deformation of a body can usually be neglected when investigating its motion. Because force has both magnitude and direction, it is a vector quantity. The representation of forces by vectors implies that they are concentrated either at a single point or along a single line. This is, however, physically impossible. On a loaded component of a structure, for example, the applied force produces an internal force, or stress , that is distributed over the cross section of the component. The force of gravity is invariably distributed throughout the volume of a body. Nonetheless, when the equilibrium of a body is the primary consideration , it is generally valid as well as convenient to assume that the forces are concentrated at a single point. In the case of gravitational force, the total weight of a body may be assumed to be concentrated at its centre of gravity ( see gravity, centre of ). Key People: Isaac Newton Christiaan Huygens Sir Charles Vernon Boys (Show more) Related Topics: electromagnetism gravity fundamental force wear friction (Show more) On the Web: Open Library Publishing Platform - College Physics – Fundamentals and Applications - Concepts: Introduction to Forces; NewtonÂ’s Laws (May 02, 2026) (Show more) See all related content Physicists use the newton , a unit of the International System (SI), for measuring force. A newton is the force needed to accelerate a body weighing one kilogram by one metre per second per second. The formula F = ma is employed to calculate the number of newtons required to increase or decrease the velocity of a given body. In countries still using the English system of measurement , engineers commonly measure force in pounds. One pound of force imparts to a one-pound object an acceleration of 32.17 feet per second squared. Britannica Quiz Physics and Natural Law The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen .