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This question offers plenty of ways to be fooled, especially if you are careless with your physics terminology. So here's a quick language lesson:
- First, the acceleration of gravity is the physics term that refers to the acceleration that all objects experience when in a state of free fall (acted upon by gravity alone). Since all objects free fall (no air resistance) at the same rate regardless of their mass, all objects have the same acceleration of gravity value.
- Second, the mass of an object is the amount of stuff possessed by an object. An object with more stuff has more mass. (And that should help you evaluate the difference in mass between an elephant and a feather.) Mass is quite different than weight. Weight is the force of gravity that acts upon the stuff that an object possesses. Consistent with the formula Weight = m•g, objects with more mass have more weight.
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Air resistance is the upward force that resists the motion of a falling object. The amount of air resistance depends upon the speed of the falling object. As an object falls faster, the amount of air resistance increases. As the upward force of air resistance increases, it will eventually become equal in magnitude to the downward pull of gravity and a balance of forces exists. When this balance of forces exists, there is no more acceleration and the object is said to have reached a terminal or final velocity value. It will keep this velocity value for the remainder of the fall.
A feather, with its very small downward force of gravity, will reach this state of terminal velocity after a few moments of fall. Having very little weight, air resistance has a significant affect upon its motion. Very little air resistance is required to bring it to a terminal velocity. The elephant has a much greater downward force of gravity. An object as massive as an elephant will have to fall for a considerable time and reach high speeds before achieving a state of terminal velocity. Having such a large force of gravity, the affect of small forces of air resistance is almost insignificant. An elephant would have to fall for a long time, accelerating to high speeds before the air resistance force is large enough to balance the force of gravity. For this reason, a more massive elephant reaches greater terminal velocity values than a less massive feather.
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