acceleration

acceleration change in the velocity of a body with respect to time. Since velocity is a vector quantity, involving both magnitude and direction, acceleration is also a vector. In order to produce an acceleration, a force must be applied to the body. The magnitude of the force F must be directly proportional to both the mass of the body m and the desired acceleration a, according to Newton's second law of motion, F = ma. The exact nature of the acceleration produced depends on the relative directions of the original velocity and the force. A force acting in the same direction as the velocity changes only the speed of the body. An appropriate force acting always at right angles to the velocity changes the direction of the velocity but not the speed. An example of such an accelerating force is the gravitational force exerted by a planet on a satellite moving in a circular orbit. A force may also act in the opposite direction from the original velocity. In this case the speed of the body is decreased. Such an acceleration is often referred to as a deceleration. If the acceleration is constant, as for a body falling near the earth, the following formulas may be used to compute the acceleration a of a body from knowledge of the elapsed time t, the distance s through which the body moves in that time, the initial velocity v_i , and the final velocity v_f :

a =( v_f² - v_i² )/2 s
a =2( s - v_it )/ t²
a =( v_f - v_i )/ t

acceleration Amount by which the velocity (speed in a particular direction) of an object increases in a certain time.

Acceleration can involve a change in speed and direction. It is measured in metres or feet per second per second (m/s² or ft/s²). For example, if an object accelerates from 20m/s² to 30m/s², it has accelerated by 10m/s². A stone dropped over a cliff accelerates from zero velocity at a rate of 9.81m (32.2ft) per second per second, this acceleration being due to the pull of Earth's gravity. The rate of acceleration can be found by applying the equation: acceleration = (change in velocity)/(time taken for change). Deceleration is a decrease in velocity. See also gravitation