Summary
Highlights
Inertia is defined as the resistance to a change in motion. An object at rest stays at rest, and an object in motion stays in motion with the same velocity unless an unbalanced force acts upon it. This explains why you feel a jerk when a car brakes abruptly.
Inertia can be demonstrated with a simple experiment using a glass, a coin, and a card. When the card is quickly pulled, the coin, due to its inertia, maintains its state of rest and falls into the glass due to gravity. This principle also applies to the classic magician's tablecloth trick.
The inertia of an object is directly dependent on its mass; more mass means more inertia. Therefore, a greater resultant force is needed to change the motion (start, stop, or change direction) of an object with higher inertia. This is evident when comparing two trolleys with different masses or when turning a sharp corner in a car, where your inertia resists the change in direction.
Inertial mass is a measure of how difficult it is to change an object's velocity. A larger inertial mass requires a greater force or produces a smaller acceleration for a given force, as described by the formula Force = Mass x Acceleration. An example is throwing a shotput versus a tennis ball; the shotput requires much more force due to its larger inertial mass.
Inertia has practical uses in mechanical devices such as inertial reel seatbelts in cars, which lock up during sudden stops, and seismometers, which detect and record earthquakes. Seismometers leverage inertia by having a suspended mass that, due to its inertia, remains relatively still while the surrounding case and paper move during an earthquake, allowing a pen to trace the tremor.