ACCELERATION, the rate of change of the velocity of a body. If the velocity of the body is constant, its acceleration is said to be zero. If the velocity increases uniformly, so that at the end of every second it is greater than it was at the end of a preceding second by a constant amount, the acceleration is said to be uniform, and the motion is said to be uni formly accelerated. If the velocity is decreas ing, the acceleration is said to be negative. A body falling freely under the influence of grav ity affords the most familiar example of uni form (or constant) acceleration. When the body falls in air or any other medium, the phenomena are complicated by the resistance of the medium; but when it falls in a vacuum its velocity increases every second by the same constant amount. Thus if the body starts from rest, it will have a velocity of 32.2 feet per second at the end of the first second, 64.4 feet per second at the end of the second second, 96.6 feet per second at the end of the third second and so on. The acceleration produced
by gravity is therefore said to be 32.2 feet per second each second; but this varies somewhat with the latitude and the height above the sea. (See FORCE OF GRAVITY). The acceleration experienced under given circumstances is pro portional to the force acting upon the body in the direction in which its motion is accelerated. Thus if the foregoing experiment with a falling body were tried upon some other planet, and we found that the velocity of the falling body was increased by 322.0 feet per second every second (instead of 32.2 feet), we. should know that the force of gravitation at the surface of that planet is precisely 10 times as great as it is upon the surface of the earth. In physics and theoretical mechanics a force is always measured by the acceleration it produces when exerted upon a unit mass. For a further account of the relation between force, mass and acceler ation, also' see FORCE.