decelerations

Decelerations refer to the reduction of speed of an object over time. In physics, deceleration is the negative of acceleration: a = dv/dt. When the magnitude of velocity decreases, the acceleration vector points opposite to the direction of motion, and its magnitude |a| represents the rate at which speed is lost. If acceleration is constant and negative, the motion is said to undergo uniform deceleration.

For one-dimensional motion with an initial velocity v0 and constant acceleration a (where a < 0), the velocity at time t is v = v0 + a t, and the position is s = v0 t + 0.5 a t^2. The time to stop is t_stop = -v0 / a, and the stopping distance is d_stop = v0^2 / (2|a|). The signs reflect the chosen coordinate system; absolute deceleration is |a|. Deceleration can thus be analyzed with the same kinematic framework used for acceleration, by focusing on the magnitude of speed reduction rather than direction alone.

Examples of deceleration occur in everyday life, such as braking a car, landing a spacecraft with propulsion, or a parachutist employing a parachute. The rate of deceleration may be expressed in meters per second squared or in g's, where 1 g ≈ 9.81 m/s^2. In engineering and safety contexts, deceleration profiles influence design choices for restraints, airbags, and structural limits to protect occupants.

Other uses of the term appear in fields like medicine; for example, fetal heart-rate decelerations describe periodic drops in heart rate during labor and can have clinical significance depending on the pattern and context.