accelerate

Other forms

Proper acceleration, the acceleration of a body relative to a free-fall condition, is measured by an instrument called an accelerometer. Newton's second law is normally applied in an inertial reference frame. In a reference frame accelerating with acceleration ${\displaystyle a}$ (in one dimension), Newton's laws can still be used by introducing an inertial force (fictitious force) ${\displaystyle F=-ma}$ on a mass ${\displaystyle m}$, opposite the acceleration of the frame. This accounts for the tendency of the mass to maintain its inertial motion—to stay "as is," at rest or moving at constant velocity—while the frame accelerates. One example is that a person in an elevator feels heavier or lighter as the elevator accelerates or decelerates. If ${\displaystyle m}$ is known, measurement of the supporting force on the mass can be used to infer the acceleration; this is the principle of a mechanical accelerometer. In general relativity, gravity and inertial acceleration may be locally indistinguishable (see General relativity).