Inertnost

Inertnost, or inertia, is the property of matter to resist changes in its state of motion or rest. The concept is tied to mass: the greater the mass, the greater the resistance to changes in velocity. The term comes from Latin inertia, meaning a lack of activity or motion.

Translational inertia describes the tendency of an object to maintain its straight-line motion or its rest state unless acted upon by an external force. This is encapsulated in Newton's first law, the law of inertia. Mass is the quantitative measure of this resistance: a heavier object requires a larger net force to achieve the same acceleration as a lighter one.

Rotational inertia, or moment of inertia, describes resistance to angular acceleration. It depends on how mass is distributed with respect to an axis. For a discrete system, I = sum m_i r_i^2; for a continuous body, I = ∫ r^2 dm. Common cases include a solid disk with I = 1/2 MR^2, a thin ring with I = MR^2, and a solid sphere with I = 2/5 MR^2. Moment of inertia determines how easily an object can change its rotational speed and influences angular momentum L = Iω and torque τ = dL/dt.

Inertial frames are reference frames in which Newton's laws hold without fictitious forces. The broader framework of dynamics uses p = mv and F = dp/dt for linear motion, and L = Iω and τ = dL/dt for rotation. Inertnost is a foundational concept in classical mechanics, explaining everyday phenomena—from why a parked car resists sudden stops to why a spinning bicycle wheel maintains orientation when midair. It is not a force but a property arising from mass and the distribution of that mass.