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![The story of Isaac Newton discovering the laws of gravity by watching apples falling from a tree is probably just a myth. He did do his work on gravity while at a farm, but that is about as much as can be proven.](http://www.scienceiq.com/Images/FactsImages/apple_falling.gif)
An object falling through the atmosphere is a good example of this principle. Just prior to release, the velocity of the object is zero, the object is at rest, and the weight of the object is balanced by some restraining device (a rope). There is no net force on the object, and the object would remain at rest indefinitely. When the rope is cut, the object is subjected to a single force, the gravitational attraction of the earth. Since there is no initial air resistance, the object begins to free fall and accelerate. But as the object velocity increases, it encounters air resistance, or drag, which opposes the motion. The magnitude of the drag depends on the square of the velocity. The drag increases until it is equal to the weight. At that point, there is no net external force on the object, the acceleration goes to zero, and the body falls at a constant terminal velocity.
The magnitude of the velocity depends on the relative magnitude of the weight, the drag coefficient, the air density, and the size of the object.