Interactions and forces

Interactions are mutual influences between physical systems; forces describe how those influences change motion or shape. Four fundamental interactions—gravitational, electromagnetic, strong, and weak—govern known physical phenomena.

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Newton's laws of motion: Three classical laws relating an object's motion to applied forces and interactions. They connect net force to changes in motion in the classical framework.
Gravity: The interaction associated with mass and energy that governs attraction and the large-scale structure of the universe. It is the long-range interaction that dominates the motion of astronomical bodies.
Electrostatics: The study of electric charges at rest and the forces and fields they produce. It explains attraction, repulsion, and charge distributions in ordinary materials.
Contact force: A force transmitted through physical contact between bodies, such as friction or a normal force. Contact forces emerge from underlying interactions rather than forming a separate fundamental interaction.
Force: A physical influence represented in classical mechanics as a push or pull that can change motion or shape. This is the central quantity used to describe mechanical interactions.
Electromagnetism: The fundamental interaction between electric charges, described by electric and magnetic fields. It governs light, electricity, magnetism, and most interactions between atoms.
Friction: A contact force that resists relative motion or attempted motion between surfaces. It is a macroscopic force arising from electromagnetic interactions between materials.
Action at a distance: The idea that one body can influence another across space without an intervening local mechanism. Field theories replace this historical picture with influences propagated through fields.
Momentum: A vector quantity equal to an object's mass multiplied by its velocity. Interactions transfer momentum, making it a useful measure of their effects.
Strong interaction: The fundamental interaction that binds quarks and, residually, protons and neutrons in atomic nuclei. Its residual effect overcomes electrical repulsion inside stable nuclei.