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Inertia

ih-NER-shuh /ɪˈnɜːrʃə/

nounC1GRESAT

technicalformal

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 not a force, but rather a fundamental property of mass. The more mass an object has, the greater its inertia, and the harder it is to change its state of motion.

The story behind the word. The word 'Inertia' comes from the Latin 'inertia', which means 'idleness', 'inactivity', or 'sluggishness'. This is derived from 'iners', meaning 'unskilled', 'idle', or 'inactive'. The scientific concept was introduced by Johannes Kepler and later formalized by Galileo and Newton, who defined it as the property of matter to resist changes in its state of motion or rest, perfectly capturing the 'idleness' of an object that doesn't want to change.

Word relationships

resistance: Inertia is a specific type of resistance to changes in motion; resistance is a broader term.
momentum: Momentum is a measure of mass in motion (mass x velocity); inertia is the property of resisting changes to that motion.

changeability: The quality of being easily changed.
dynamism: The quality of being characterized by vigorous activity and progress.

Commonly confused with

Momentum: Inertia is the resistance to change in motion; momentum is the quantity of motion an object possesses.

Word family

inert (adjective): lacking the ability or strength to move; chemically inactive

Collocations

moment of inertia
thermal inertia
resist inertia
overcome inertia

Idioms & expressions

inertia of habit

Example sentences

"Due to inertia, a passenger in a car continues to move forward when the car suddenly stops." Explaining a physical principle.
"Once a spacecraft reaches its cruising speed in the vacuum of space, it can travel vast distances without expending fuel, thanks to inertia." In space.
"A bowling ball has more inertia than a tennis ball, making it harder to get moving and harder to stop once it is in motion." Relating to mass.

Memory hook

Think of 'INERTia' as something that wants to stay 'INERT' (inactive) or keep doing what it's already doing. Resistance to change

When not to use

While 'inertia' can be used metaphorically to describe a lack of activity or resistance to change (e.g., 'political inertia'), ensure the context is clear if you intend the scientific meaning.

Fun facts

Newton's First Law of Motion is often called the Law of Inertia.
The concept of inertia is why seatbelts are so important in vehicles.

Related words

Motion, Force, Acceleration

Explore more words

propel, equilibrium, Properties, stability, accretion, perpetual