2 ms·

Buoyancy is an effect of gravity. Gravity pulls down on the chair harder than on the air, that's why the chair sinks to the floor. Since air is a fluid, it pus

by sp332 4y ago

Buoyancy is an effect of gravity. Gravity pulls down on the chair harder than on the air, that's why the chair sinks to the floor. Since air is a fluid, it pushes equally hard on every square inch of the chair, including the ones facing upward (on the bottom). So I think that's why they would say there is no net force exerted by the air. But that is technically slightly wrong, since there are small spots where the legs or wheels of the chair are touching the floor and not the air. So the net effect from the air would be just a little bit downward.

robocat 4y ago
> Gravity pulls down on the chair harder than on the air That is usually not the way to express what you mean? Drop a feather and a brick on the moon and they take the same time to fall because gravity pulls down on them the same amount. But yeah, the actual forces on a brick and feather are different. There is a pressure gradient from the top of the atmosphere to the bottom. So voluminous objects have more pressure pushing up on the bottom of an object than they do pushing down from the top - causing buoyancy (assuming the object is mostly surrounded by air). The buoyancy in this case is offset by gravity pulling down on the mass of the chair. I think the question is a kind of trick question because it ignores buoyancy as a force, but the forces definitely exist. There is definitely a downward force by the air column above the chair, and there is definitely an upward force by the air below the chair (and also on the sides, but that nets out equally). 101kPa is significant! The trick is that I think technically “A net downward force exerted by the air” on the chair exists as a negative number. But the question is not asking as though a physicist is answering precisely, even though it is asking precise questions about forces in physics. Annoying question with a trick - the worst kind - the kind of test question where you have to answer it “correctly”, not correctly. Any of the above could be wrong or poorly explained, but that is what I think after reading the comments and thinking about it a little more.

by sp332 4y ago

robocat 4y ago

> Gravity pulls down on the chair harder than on the air That is usually not the way to express what you mean? Drop a feather and a brick on the moon and they take the same time to fall because gravity pulls down on them the same amount. But yeah, the actual forces on a brick and feather are different. There is a pressure gradient from the top of the atmosphere to the bottom. So voluminous objects have more pressure pushing up on the bottom of an object than they do pushing down from the top - causing buoyancy (assuming the object is mostly surrounded by air). The buoyancy in this case is offset by gravity pulling down on the mass of the chair. I think the question is a kind of trick question because it ignores buoyancy as a force, but the forces definitely exist. There is definitely a downward force by the air column above the chair, and there is definitely an upward force by the air below the chair (and also on the sides, but that nets out equally). 101kPa is significant! The trick is that I think technically “A net downward force exerted by the air” on the chair exists as a negative number. But the question is not asking as though a physicist is answering precisely, even though it is asking precise questions about forces in physics. Annoying question with a trick - the worst kind - the kind of test question where you have to answer it “correctly”, not correctly. Any of the above could be wrong or poorly explained, but that is what I think after reading the comments and thinking about it a little more.