It is normally said that mass and weight are the same on earth, but that is not true. In a vacuum on earth an object's mass would be the same as its weight, but air provides some buoyancy that lessens the weight, similar to what happens when something is weighed under water. If a person weighs 150 pounds, what would he weigh without the effect of the air? Does it make a significant difference?
2006-12-12 03:26:02 · 5 answers · asked by sofarsogood 5 in Science & Mathematics ➔ Physics
You are right that air provides some buoyancy, even if it's not much compared to water. Buoyancy is not significant for objects (such as people) who are very dense compared to air. Most people have a density pretty close to water (which is why people usually float, but not by much---after all, we are mostly water). Suppose someone weighs 150 pounds. Therefore, they have a mass of about 70kg. Since 1000 kg of water is about 1 cu. meter, that person displaces about 0.07 cu. meter of air. Room temperature air has a mass of about 1.2 kg/cu. meter, so the mass of the air displaced by the person is about 0.084 kg. That's equivalent to about 0.2 pounds. Therefore, the buoyancy makes little difference, even though it might be enough to affect the number a precise-enough scale gives you. (As a rule of thumb, you could say a person's buoyancy in air is just over 0.1% of their weight.)
Buoyancy in air, however, is not insignificant for hot-air balloons, helium party balloons, and the Goodyear blimp. In each case, a gas with a density considerably less than air (either hot air or helium) allows for the buoyant force to be significantly greater than the surrounding air, which allows for the object to rise, even bringing along heavier components of the object (rubber, string, people, etc.)
2006-12-12 03:44:03 · answer #1 · answered by Minnesota_Slinger 3 · 0⤊ 0⤋
Not true...bouyancy does not lessen weight, it acts against weight. A ship in water is the same weight as out of the water. Bouyancy is a force that acts in the opposite direction as weight. Air would not reduce an objects mass because of bouyancy. Air pressure acts on all sides of the object. Simply speaking, air pushing down on you is equal to the air pushing up on you, so weight, and mass for that matter, is not effected. Mass and weight are not the same property, anywhere. Mass is the amount of material in an object, weight is the force in which gravity acts on that mass. Mass of an object is the same on the moon as on earth, but the weight is about 1/7th that on earth because the moon's gravity is 1/7th that of earth.
When you weigh something under water, you must contend with bouyant forces, which could skew your results. In air, the density of air is so low that the bouyant forces have very little interference with the measurements. Although there are some cases, as like trying to weigh a balloon, where you would need another technique to measure weight as opposed to a simple place it on the scale measurement. Interesting question...
2006-12-12 03:42:08 · answer #2 · answered by Mike B 2 · 0⤊ 1⤋
The buoyancy of air does not make a significant difference. Air is significantly less dense than water so the buoyant force in air is very small. Your person might weigh 149.999 lbs. when this buyonat force is taken into account.
2006-12-12 03:32:25 · answer #3 · answered by msi_cord 7 · 0⤊ 0⤋
On the moon, which of these weighs more? a pound of feathers or a pound of water.
2006-12-12 04:00:10 · answer #4 · answered by Alan B 2 · 1⤊ 1⤋
ask taddfgb
2006-12-12 03:45:57 · answer #5 · answered by dodo 4 · 0⤊ 3⤋