Acceleration is the rate at which an object’s velocity changes over time. Velocity is the speed and distance traveled by an object, while displacement is the distance traveled. Newton’s second law links force, mass, and acceleration. Gravity causes constant acceleration, and all objects on Earth experience a rate of 9.81 ms2. The difference in motion between objects falling is due to air resistance, not mass.
Constant acceleration is when the rate at which a body’s speed or velocity changes — how much it’s speeding up or slowing down — stays the same over a set amount of time. Acceleration, velocity and displacement are the three main quantities that describe the motion 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 given period of time. Acceleration is the measure of the change in velocity, i.e. how the velocity of a body changes at a given moment.
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 constant acceleration. The metric unit of force is called a 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 rest point, after one second, the body will be moving at 3.28 feet (1m) per second, and after 10 seconds it will be moving at 32.8 feet (10m) per second.
Any object affected by gravity experiences constant acceleration. Newton supposedly came up with his theory of gravity when an apple landed next to him while he was sitting under a tree. It is not known whether this story is true, but what is known is that Newton deduced that an apple falls due to 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 fell faster than lighter objects. A heavy stone falls faster than a light feather, but this has nothing to do with their respective masses. Calculating the acceleration caused by gravity on Earth depends on the object’s radius, its mass, and a number known as the gravitational force constant. On the Earth’s surface, all objects experience a constant rate of acceleration of about 9.81 ms2, although this changes slightly with latitude because the Earth is not a perfect sphere.
The differences in motion between a feather and a rock as they fall are caused by the frictional force of the air particles acting on them. This force opposes gravity and is the result of their shapes rather than their masses. A feather experiences much more friction with the air than a rock. On the Moon, where there is no atmosphere, both objects would fall at the same speed.
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