acceleration

Rate of change of the velocity of a moving body. It is usually measured in feet per second per second (ft s⁻²) or meters per second per second (m s⁻²). Because velocity is a vector quantity (possessing both magnitude and direction) a body travelling at constant speed may be said to be accelerating if its direction of motion changes. According to Newton's second law of motion, a body will accelerate only if it is acted upon by an unbalanced, or resultant, force. Acceleration due to gravity is the acceleration of a body falling freely under the influence of the Earth's gravitational field; it varies slightly at different latitudes and altitudes. The value adopted internationally for gravitational acceleration is 32.174 ft s⁻²/9.806 ms⁻².

The average acceleration a of an object travelling in a straight line over a period of time t may be calculated using the formula: a = change of velocity/t or, where u is its initial velocity and v its final velocity: a = (v − u)/t. A negative answer shows that the object is slowing down (decelerating). See also equations of motion.