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inertia

ih-NUR-shuh /ɪˈnɜːrʃə/

nounB2SATACTTOEFLIELTS

GeneralTechnicalAcademic

A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.

Inertia is a passive property of mass. It is not a force itself, but rather a resistance to any change in velocity (speed or direction). A heavy object has high inertia, meaning it is hard to start moving, and once moving, it is hard to stop or turn.

The story behind the word. Coined in its modern scientific sense by Johannes Kepler in the 17th century, inertia comes from the Latin word 'iners', which meant 'sluggish', 'idle', or 'unskilled' (literally 'without art/skill'). Kepler used it to describe the natural laziness of matter—its tendency to do absolutely nothing unless forced.

Word relationships

resistance to change: 'Resistance to change' is a conceptual explanation, whereas inertia is the precise physical property quantified in physics.

susceptibility to acceleration: An object with low inertia is highly susceptible to acceleration by small forces.

Commonly confused with

momentum: Inertia is an inherent property of mass regardless of motion, while momentum is the product of mass and velocity (it only exists when the object is moving).

Word family

inert (adjective): Lacking the ability or strength to move; chemically unreactive.
inertial (adjective): Relating to or arising from inertia.

Collocations

moment of inertia
rotational inertia
thermal inertia
overcome inertia

Example sentences

"Because of inertia, passengers in a car slide forward when the driver slams on the brakes." A physics textbook explaining Newton's laws
"The heavy steel flywheel has high rotational inertia, which helps smooth out the power pulses of the internal combustion engine." An engineering analysis of a flywheel

Memory hook

Think of 'inert' gases like helium that do not react with anything. 'Inertia' is the physical equivalent of being too lazy to change what you are doing. Inertia is the physical world's laziness.

When not to use

Do not use inertia to describe active resistance or friction; it is strictly the passive resistance of mass to acceleration.

Fun facts

The 'moment of inertia' is the rotational equivalent of mass; it explains why it is easier to spin a figure skater when her arms are pulled close to her body than when they are extended.
Seatbelts are designed specifically to counteract your body's inertia during a sudden car crash.