Deceleration

Example

When a vehicle starts from a standstill (zero velocity, in an inertial frame of reference) and travels in a straight line at increasing speeds, it is accelerating in the direction of travel. If the vehicle turns, an acceleration occurs toward the new direction and changes its motion vector. The component of acceleration of the vehicle in the direction of its velocity is called a linear acceleration or tangential acceleration, its effect being the passengers on board experience as a fictitious force pushing them back into or away from their seats. When changing direction, the component of the acceleration perpendicular to the velocity is called radial or normal acceleration (or centripetal acceleration during circular motion), the reaction to which the passengers experience as a centrifugal force (another fictitious force). If the speed of the vehicle decreases, this is an acceleration in the opposite direction of the velocity vector, sometimes called deceleration or retardation, and passengers experience the reaction to deceleration as an inertial force pushing them forward. Such deceleration is often achieved by retrorocket burning in spacecraft. Both acceleration and deceleration are treated the same, as they are both changes in velocity. Each of these accelerations (tangential, radial, deceleration) is felt by passengers until their relative (differential) velocity is neutralised in reference to the acceleration due to change in speed.