decelerating

Decelerating is the process of reducing the speed of an object. In physics, deceleration is typically described as acceleration in the direction opposite to the object's velocity, i.e., a negative acceleration when speed is decreasing in the chosen direction. More precisely, acceleration is the time rate of change of velocity; if the speed (the magnitude of velocity) decreases, the projection of acceleration along the velocity is negative.

Deceleration occurs due to forces opposing motion: friction, aerodynamic drag, gravity when moving uphill, braking forces in vehicles, or thrust reversal in rockets. In everyday language, deceleration is often called braking or slowing down.

In straight-line motion with constant acceleration, speed decreases according to v^2 = v0^2 + 2 a s, where a is negative for deceleration. Stopping distance is the sum of reaction distance and braking distance. The magnitude of deceleration is measured in meters per second squared (m/s^2) or g's (1 g ≈ 9.81 m/s^2). Larger deceleration values imply a shorter stopping time but greater forces on the moving body.

Deceleration can be relevant in safety analysis, vehicle design, and athletics. It can occur without a loss of contact with a surface, such as slowing due to friction, or in circular motion where speed remains constant but velocity changes direction; in that case the acceleration is centripetal, not a deceleration of speed.

In summary, decelerating means slowing down, achieved by forces that oppose motion; in physics, it corresponds to negative acceleration with respect to the direction of travel.