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Constant acceleration is when the rate at which the speed or velocity of a body is changing — how much it is either speeding up or slowing down — remains the same during a given period of time. Acceleration, velocity and displacement are the three principle quantities that describe the movement of a body. Displacement is the measure of the distance traveled by a body. Velocity describes how fast the body is traveling, how much distance it is covering during a specific period of time. Acceleration is the measure of changing velocity, that is, how the speed of a body is changing at a particular time.
Isaac Newton was the first to link acceleration with force. Newton's second law of motion states that the force acting on a body (F) can be described in terms of its mass (m) and its acceleration (a) by the equation F=ma. This means that a body subjected to a constant force will experience a constant acceleration. The metric unit of force is called the Newton (N), and a force of 1 Newton will cause a 2.2-pound (1 kg) body to accelerate at 3.28 feet (1 m) per second per second (ms2). This means that, from a point of rest, after one second, the body will be moving at 3.28 feet (1 m) per second, and after 10 seconds, it will be moving at 32.8 feet (10 m) per second.
Any object that is influenced by gravity is undergoing constant acceleration. Newton supposedly devised his theory of gravity when an apple landed next to him while he was sitting under a tree. Whether this story is true is not known, but what is known is that Newton deduced that an apple falls because of a force acting between the Earth and the apple, and that this force is the result of their respective masses.
Before Newton, it was generally thought that heavier objects fall more quickly than lighter objects. A heavy rock falls more quickly than a light feather, but this has nothing to do with their respective masses. The calculation of acceleration caused by gravity on Earth depends on the object's radius, its mass and a number known as the gravitational force constant. At the Earth's surface, all objects undergo a rate of constant acceleration of about 9.81 ms2, although this changes slightly with latitude because the Earth is not a perfect sphere.
Differences in motion between a feather and a rock as they fall are caused by the frictional force of air particles acting upon them. This force opposes gravity and is the result of their shapes rather than their masses. A feather is subject to far more air friction than a rock. On the Moon, where there is no atmosphere, both objects would fall at the same rate.