Acceleration

is when the speed at which an object is traveling is further increased. The object travels faster because some force is applied to push or pull the object, making it move more quickly. In physics, when calculations are made concerning acceleration, it is usually represented by the symbol α and is measured in metres per second, per second or metres per second squared (written in mathematical shorthand as ms^-2).

When bodies fall freely under the influence of the force of GRAVITY they all fall at the same rate, that is at a uniform acceleration. Whether the object is a lead weight or a feather they fall at the same rate due to gravity. The reason that in reality the lead weight hits the ground before the feather is due to air resistance holding back the feather. Acceleration due to gravity is approximately 9.8 metres per second per second so when the air resistance is very little, the velocity (speed) of a falling object increases by 9.8 metres per second, per second. On the other hand, an object shot straight upwards with a particular velocity will slow down (decelerate) by the same rate every second until it reaches its greatest height after a particular period of time. When this maximum height is reached, the object that had been shot straight upwards (no horizontal motion whatsoever) will start to fall with an acceleration of 9.8 ms-2.

Taken from Dictionary of Science

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In physics, and more specifically kinematics, acceleration is the change in velocity over time. Because velocity is a vector, it can change in two ways: a change in magnitude and/or a change in direction. In one dimension, i.e. a line, acceleration is the rate at which something speeds up or slows down. However, as a vector quantity, acceleration is also the rate at which direction changes. Acceleration has the dimensions L T⁻². In SI units, acceleration is measured in metres per second squared (m/s²).

In common speech, the term acceleration commonly is used for an increase in speed (the magnitude of velocity); a decrease in speed is called deceleration. In physics, a change in the direction of velocity also is an acceleration: for rotary motion, the change in direction of velocity results in centripetal (toward the center) acceleration; where as the rate of change of speed is a tangential acceleration.

In classical mechanics, for a body with constant mass, the acceleration of the body is proportional to the resultant (total) force acting on it (Newton's second law):

where F is the resultant force acting on the body, m is the mass of the body, and a is its acceleration.

Taken from Wikipedia