Inertia
Inertia is the tendency of an object to resist changes in its state of rest or uniform motion. Mass is a measure of inertia.
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Definition of inertia
Inertia is the property of matter by which it tends to remain at rest or in uniform motion in a straight line unless acted upon by an external force. It is a qualitative concept expressed quantitatively by mass: the greater the mass, the greater the inertia.
Mass as measure of inertia
Mass is the measure of inertia. A heavier object is harder to accelerate or decelerate than a lighter one when the same force is applied (from F = ma, larger m means smaller a for the same F). In everyday life we feel inertia when a vehicle brakes suddenly and we lurch forward.
Examples
When a bus starts moving, passengers feel pushed backward (their bodies tend to stay at rest). When the bus stops, passengers lurch forward (their bodies tend to keep moving). Seat belts and airbags help by applying a force over time to reduce the effect of inertia in collisions.
Why does a heavy truck take longer to stop than a car moving at the same speed?