Motion

Motion is the change in position of a physical object with respect to time, described relative to a chosen frame of reference. An object is in motion if its position changes over time; otherwise, it is at rest. The concept applies across scales and to waves and fields as well as objects.

Kinematics is the branch of mechanics that analyzes motion without regard to its causes. Core quantities are displacement (a vector describing position change), velocity (the rate of change of displacement), and acceleration (the rate of change of velocity). In one dimension, with constant acceleration, the equations s = s0 + v0 t + 1/2 a t^2 and v = v0 + a t relate position and velocity to time. In general, velocity and acceleration are vector quantities that depend on the chosen coordinate system.

Motion is often categorized as translational, where an object moves through space, and rotational, where motion occurs about an axis. Rotational motion uses angular velocity and angular acceleration, with torque and moment of inertia describing the rotational response. Many real-world motions combine these forms, and oscillatory motion, such as that of springs and pendulums, is also common.

Dynamics connects motion to causes through forces. Newton's laws relate forces to changes in motion, with forces arising from gravity, contact, tension, and other interactions. Energy concepts describe how work changes kinetic energy, and potential energy and momentum conservation help explain system behavior across diverse situations.

Classical descriptions typically assume inertial frames of reference. At high speeds or strong gravitational fields, relativistic and quantum considerations become important, leading to more general theories, but everyday motion is often well described by Newtonian dynamics.