When we think of bodies that fall from a great height, we expect heavy bodies to fall faster than light bodies. Actually they all fall with the same velocity, contrary to human intuition.
Mass Weight Difference: To an average person it appears that bodies simply fall due to their weight. There seems to be no other reason why they fall. After all feathers and kites which are very light tend to remain in the air for a long period of time. This is where intuition fails to give us accurate information. Actually bodies do not fall because of their weight. If they were taken to space, they would become weightless. All bodies, whether a feather, or a 1000 ton ship would be weightless in space. There is no absolute quantity like weight, and thus weight is not the factor that propels them towards the earth.
The absolute quantity that is their in a body is its "mass". A feather has less mass than a steel ball because it contains less material. The more the material packed in a body the more is its mass. If the same force is applied to two bodies, the body with the small mass will have more acceleration than the body with more mass. That is why we are able to throw a small stone to a great distance while we cannot throw a heavy stone the same way. The more the mass, the more will be the force required to set it in motion.
All bodies are weightless in space, whether it is a feather or a steel ball. However, once they are brought closer to the earth, the earth begins to attract it. The less the mass of the body, the less the attraction and the greater the mass of the body the greater the attraction. This attraction is directed towards the center of the earth. This is why a body with a greater mass is difficult to lift because you are trying to move it away from the center. Since a body with a greater quantity of material inside it (such as a steel ball as opposed to a feather) is attracted to the center of the earth with greater force, that body seems to have greater "weight". Thus the weight of a body is simply the force with which the earth attracts it. The larger the mass inside it, the more the attraction, and the more the weight that seems to us.
Falling Bodies: Coming now to falling bodies, as we said before, each body needs force in PROPORTION to its mass to attain the same velocity. A light body needs less force and a massive body needs more force. It just so happens that a light body is attracted to the core of the earth with a lesser force and a heavy body is attracted with proportionally greater force. The proportion is just enough to impart the same velocity to all the bodies, light or massive, that are freely dropped from the same height. That is why bodies of different masses dropped freely from the same height fall with the same velocity (when the upward air-resistance is ignored).
Galileo Galilee is the first person who demonstrated this counter-intuitive matter. It is said that he arrived at the deduction after he dropped countless spherical balls of differing weights from the leaning tower of Pisa. The story might be a myth, but the discovery by him is a fact.
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Picture Credit: Leaning Tower (Creative Commons)