Acceleration
The term acceleration, used in physics, is a vector quantity. This means that acceleration contains both a number which is called its magnitude and a specific direction. An object is said to be accelerating if its rate of change of velocity is increasing or decreasing over a period of time and/or if its direction of motion is changing. The units for acceleration include a distance unit and two time units. Examples are m/s2 and mi/hr/s. Sir Isaac Newton in his Second Law of Motion defined acceleration as the ratio of an unbalanced force acting on an object to the mass of the object.
The study of motion by Galileo Galilei (1564-1642) in the late sixteenth and early seventeenth centuries and by Sir Isaac Newton (1642-1727) in the mid-seventeenth century was one of the major cornerstones of modern Western experimental science. Over a period of 20 years, Galileo observed the motions of objects rolling down various inclines and attempted to time these events. He discovered that the distance an object traveled was proportional to the square of the time that it was in motion. From these experiments came the first correct concept of accelerated motion. Newton wanted to know why acceleration occurred. In order to produce a model that would help explain how the known universe of the seventeenth century worked, Newton had to give to science andphysics the concept of a force which was mostly unknown at that time. With his Second Law of Motion, he clearly demonstrated that acceleration is caused by an unbalanced force (commonly called a push or a pull) acting on an object. What we call gravity, Newton showed was nothing more than a special type of acceleration. The interaction of the acceleration of gravity on the massof our body produces the force which is called weight. A general definition of mass is that it refers to the quantity of matter in a body.
An object that is moving in a straight line is accelerating if its velocity (sometimes incorrectly referred to as speed) is increasing or decreasing during a given period of time. Acceleration (a) can be either positive or negative depending on whether the velocity is increasing (+a) or decreasing (-a). An automobile's motion can help explain linear acceleration. The speedometer measures the velocity. If the auto starts from rest and accelerates to 60 mph in 10 seconds, what is the acceleration? The auto's velocity changed 60 mph in 10 seconds. Therefore, its acceleration is 60 mph/10 s = + six mi/hr/s. That means its acceleration changed six miles per hour every second it was moving. Notice there are one distance unit and two time units in the answer. If the auto had started at 60 mph and then stopped in 10 seconds after the brakes were applied, the acceleration would be = -6 mi/hr/s. If this automobile changes direction while moving at this constant acceleration, it will have a different acceleration because the new vector will be different from the original vector. The mathematics of vectors are quite complex.
In circular motion, the velocity may remain constant but the direction of motion will change. If our automobile is going down the road at a constant 60 mph and it goes around a curve in the road, the auto undergoes acceleration because its direction is constantly changing while it is in the curve. Roller coasters and other amusement park rides produce rapid changes in acceleration (sometimes called centripetal acceleration) which will cause such effects as "g" forces, "weightlessness" and other real or imaginary forces to act on the body, causing dramatic experiences to occur. The astronauts experience as much as 7 "gs" during lift-off of the space shuttle but once inorbit it appears that they have lost all their weight. The concept of "weightlessness" in space is a highly misunderstood phenomena. It is not caused by the fact that the shuttle is so far from the Earth; it is produced because the space shuttle is in free fall under the influence of gravity. The shuttle is traveling 17,400 mph around the Earth and it is continually falling toward the Earth, but the Earth falls away from the shuttle at exactly the same rate.
Before the time of Sir Isaac Newton, the concept of force was unknown. Newton's Second Law was a simple equation and an insight that significantly affected physics in the seventeenth century as well as today. In the Second Law, given any object of mass (m), the acceleration (a) given to that object is directly proportional to the net force (F) acting on the object and inversely proportional to the mass of the object. Symbolically, this means a = F/m or in its more familiar form F= ma. In order for acceleration to occur, a net force must act on an object.
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