A Freely Falling Body; Resistance of the Air. -A
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-39
2025-10-22
44
body that is moving under the influence of gravity alone is a freely falling body. This condition can be obtained only in a vacuum, as the air constantly offers a resistance to the passage of any body through it.
Demonstrations. - Trim a piece of stiff paper and a cork until each has the same weight as a shot or a bicycle ball. Drop all three from the same height at exactly the same time, and notice when they strike the floor. Since they all have the same weight, the force tending to give them motion is the same, but as they present different amounts of surface to the air, the resistance of the air varies. If the sheet of paper is let fall when it is flat, it will slide down on the air in various directions, but if a small part of its width is turned up at an angle, it will fall very steadily. Drop two balls of the same size, one of brass and one of wood, from a height of 20 ft. or more. `They reach the ground at practically the same time.
These demonstrations indicate in a simple way the method used by Galileo to settle by experiment the discussion which had been vigorously carried on between those who thought that the velocity of a freely falling body was proportional to its weight and those who, like Galileo, thought it was the same for all bodies. The experiment was made by dropping various bodies from the top of the leaning tower of Pisa, and showed that Galileo was right. The pull of the earth on a two-pound weight is twice its pull on a one-pound weight, but in causing the weights to fall this pull must do twice as much work on the two-pound weight as on the one-pound weight; hence the speed of the fall is the same. The resistance of the air prevents raindrops from acquiring a high velocity. A parachute likewise falls slowly because it presents a surface that is large in proportion to the weight.
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