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After I have viewed two experiments the first about Newton's law which involves a brick block and feather letting them fall one time in a vacuumed room they fall at the same time and another time in normal conditions the brick falls first. on the other hand, the second experiment illustrating free fall involves two balls , a basket ball and a heavy medical ball, in normal conditions(presence of air and gravity) they fall at the same time! So, I'm puzzled why the two balls fall together although they are of different weights and masses while the brick and the feather fall one after the other in case of throwing them in air. Is it the density or volume that make sense here. Please, I need an implementation for this issue. Thanks in advance

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    $\begingroup$ The balls don't actually fall at the same rate, but the difference is small enough that you can't see it for such a short fall. Drop them from the top of a skyscraper and you'll see the difference. $\endgroup$ Commented Feb 6, 2016 at 20:29

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The problem is air. If something is lightweight compared to its surface area, like a feather, snowflake, dust mote, animal hair, it cannot fall as fast as a brick because air resistance slows it down. Even a brick or a bowling ball eventually reaches a maximum speed when falling in air.

Any discussion of this has to assume (ideally) no air resistance, or at least that things are falling for such a short time that they never reach maximum (called terminal) velocity.

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  • $\begingroup$ In a mathematical model, without fluctuations in air resistance and constant acceleration due gravity, an object will never reach terminal velocity in finite amount of time. Thus it might be better to rephrase the last sentence to something like that in the used timescales the reached velocities will only induce an aerodynamic drag which is negligible compared to the resulting force acting on the object. $\endgroup$
    – fibonatic
    Commented Feb 7, 2016 at 0:27
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The basketball and the medicine ball are both round, and though medicine balls can be a little larger than basketballs, the surface area and shape that each presents to air resistance is roughly equivalent. The major difference is weight. Basketballs weigh in the neighborhood of 20 ounces, but medicine balls can weigh up to 24 pounds.

The experiment you witnessed in which both the medicine ball and the basketball fell at approximately the same rate (through a short distance), is an approximate illustration that weight makes no difference to the effect of gravitational acceleration on objects near the surface of the Earth if their air resistance is the same.

But the airfoil of a brick and the airfoil of a feather are vastly different. A feather provides lift to keep a bird airborne. A brick has little air resistance. The experiment you witnessed in which each fell at the same rate through a vacuum (the absence of air resistance) is another illustration that weight makes no difference to the effect of gravitational acceleration on an object in the absence of air resistance.

Free fall is the downward movement of an object under the force of gravity, only. Both experiments illustrated the effect of free fall in the Earth's gravitational field.

Newton's Law of Universal Gravitation states that every object in the universe attracts every other object along a line connecting their centers of mass, with force directly proportional to the product of their masses, and inversely proportional to the square of the distance between them. But since the mass of the Earth is so much larger than the mass of a brick, a feather, a medicine ball, or a basket ball, and since the distance between the Earth and each of those objects in the experiments you observed was tiny, the formula for universal gravitational acceleration is not applied in such cases.

Universal gravitational acceleration (big "G") is used when computing the attraction of celestial objects. Gravitational acceleration near the surface of the Earth (little "g") is about 9.8 meters per second, per second. Little "g" is used to compute the trajectories of small objects near the Earth's surface, and generally is adjusted for air resistance and wind speed.

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