massinertia

Mass inertia, or inertia, is the property of matter that resists changes to its motion or rest. In classical mechanics, inertia explains why a stationary object remains at rest and a moving object tends to keep moving in a straight line unless a net external force acts on it. The degree of resistance to linear acceleration is quantified by inertial mass, which appears in Newton's second law: F = m a. A larger mass requires a larger force to achieve the same acceleration.

In rigid bodies there is also rotational inertia, or the moment of inertia, which measures resistance to angular acceleration about an axis. Torque τ produces angular acceleration α according to τ = I α, with I = ∑ m r^2 for discrete particles or I = ∫ r^2 dm in continuous bodies. Together with linear inertia, rotational inertia governs how objects respond to forces and torques in both translation and rotation.

Inertial frames of reference are those in which Newton's laws hold. The inertial mass is the factor that relates applied force to acceleration; gravitational mass is the factor that determines gravitational attraction. The equivalence principle states these two masses are experimentally indistinguishable, a cornerstone of general relativity, where gravity is described as the curvature of spacetime rather than a force.

In practice, inertia governs everyday motion and engineering: it affects vehicle dynamics, satellite trajectories, and the behavior of rotating machinery. In high-speed physics, relativistic effects modify the relationship between force, mass, and acceleration, but the concept of inertia remains central: a body's resistance to change in motion is proportional to its mass.

Historically, Galileo’s experiments and Newton’s laws formalized inertia as a universal property of matter, later integrated into the broader framework of classical and modern physics.