inertia

Meaning: Inertia is the tendency of an object to keep its current velocity, whether it is at rest or moving in a straight line at constant speed. Only a net external force changes that velocity. Mass is the quantitative measure of inertia: the larger the mass, the harder it is to speed up, slow down or turn the object. This is Newton's first law. The rotational counterpart is moment of inertia, which depends on the mass and on how far it sits from the axis.

Equation: a = Fnet/m, where a is acceleration (m/s²), Fnet is the net force (N) and m is mass (kg). For rotation, I = Σmr², where I is moment of inertia (kg·m²), m is each piece of mass (kg) and r is its distance from the axis (m).

On the MCAT: Expect a passenger lurching forward when a car brakes, a coin dropping into a cup when a card is flicked away, or two objects of different mass given the same net force (the larger mass accelerates less). Zero net force means constant velocity, not necessarily rest.

Common confusion: Inertia is not a force and does not push an object forward. The passenger keeps moving because no force stops them as quickly as the car stops. Inertia is also not weight. Mass stays the same everywhere, while weight (mg) depends on the local g, so an object far from any planet still has the same inertia.