Mechanical Engineering

The rate of change of velocity is known as acceleration. 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 term acceleration is a vector quantity. This means that acceleration contains both magnitude and a specific direction. The units for acceleration include a distance unit and two time units. Examples are m/s2 and mi/hr/s. Sir Isaac Newton (1642-1727) in his second law of motion defined acceleration as the ratio of force acting on an object to the mass of the object.


Over a period of 20 years, Galileo Galilei (1564-1642) 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, with his second law of motion 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 mass of 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.

Linear acceleration:

An object that is moving in a straight line is accelerating if its velocity 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). If the auto starts from rest and accelerates to 20 m/s in 10 seconds, what is the acceleration? The auto’s velocity changed 20 m/s in 10 seconds. Therefore, its acceleration is 20 m/s/10 s = +2 m/s2. That means its acceleration changed two miters every second it was moving. Notice there are one distance unit and two time units in the answer. If the auto had started at 20 m/s and then stopped in 10 seconds after the brakes were applied, the acceleration would be = -2 m/s2.

Circular acceleration:

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 velocity of 20 m/s 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.

Astronauts experience as much as 7“gs” during lift-off of the space shuttle but once in orbit it appears that they have lost all their weight. It is produced because the space shuttle is in free fall under the influence of gravity. The shuttle is traveling 17,400 MPH around Earth and it is continually falling toward Earth, but the Earth falls away from the shuttle at exactly the same rate.

Force and acceleration:

Before the time of Sir Isaac Newton, the concept of force was unknown. Newton’s second law was a simple equation that significantly affected physics. 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.