Test your buoyancy understanding!

[DevonDiver]

 

[drinksfromtap]

Yeah, it would be more correct to say you have 200 lb-mass of mass under water. The problem with "imperial" units is that "pounds" are used interchangeably for mass and force at the surface of the earth. That is, a 200 lb-mass person standing at sea level is pulled toward the center of the earth with 200 lb-force of gravity. It's a lot easier to understand in metric -- pounds-mass are equivalent to kilograms, and pounds-force are equivalent to newtons (mass in kg times acceleration).

The Denver question is tricky. Since they are higher up, the gravitational force will be slightly less. This is because the gravitational force between two objects decreases as the distance between the center of the objects increases. I think you're right that the lower air density would tend to increase their weight due to buoyancy. The question then becomes whether the decrease in weight from gravitational effects is greater or less than the effect of the change of buoyancy -- anyone out there know which would "win"?

OK, I agree. We're basically discussing the word "weight". I see what you say and agree with you that the weight of an object is a combination of its mass, it's buoyancy, how it's affected by gravity, density of what's around it (water, air, etc.), etc. So weight can change, though mass doesn't. I guess most lay people like myself think of weight as the same as mass. Ei: If I'm walking on land, or scuba diving, or taking a space walk, I still "weigh" 200 pounds, even if a scale may or may not say so. Or would it be more correct to say I still have 200 lbs. of mass? I assume people living in Denver would "weigh" a bit more because the atmosphere is a little less dense.

 

[DaleC]

Some of you guys are way over thinking this.

Depends who's tied to the rock and how long they can tread water.

 

[Rylz]

The level of the pool stays the same because the water flows through the drainage system and gets pumped back in.

 

[windapp]

A floating object (or system of objects in this case) displaces a volume of water equivilant to its weight. Whether the rock is in the canoe or suspended from the canoe, the weight of the system is constant, and the volume of water displaced is the same. The level of water is therefore the same.

When the rock is seperated from the system either by cutting the rope or throwing the rock in, the displacement of water (indicated by the level) decreases by a volume equivilant to the weight of the rock. At the same time, the sunk rock increases the displacement of water by its own volume (or in the case of the suspended rock, doesn't decrease the displacement fully). Since the rock is denser than water, the displacement increase is less than the decrease due to the change in weight of the floating system.

The level therefore drops by exactly the same amount in both cases.

 

[Guba]

Yeah, it would be more correct to say you have 200 lb-mass of mass under water. The problem with "imperial" units is that "pounds" are used interchangeably for mass and force at the surface of the earth. That is, a 200 lb-mass person standing at sea level is pulled toward the center of the earth with 200 lb-force of gravity. It's a lot easier to understand in metric -- pounds-mass are equivalent to kilograms, and pounds-force are equivalent to newtons (mass in kg times acceleration).

The Denver question is tricky. Since they are higher up, the gravitational force will be slightly less. This is because the gravitational force between two objects decreases as the distance between the center of the objects increases. I think you're right that the lower air density would tend to increase their weight due to buoyancy. The question then becomes whether the decrease in weight from gravitational effects is greater or less than the effect of the change of buoyancy -- anyone out there know which would "win"?

Gravity.

I haven't done the math, but there was a study (I believe with NASA support) that used incredibly accurate measuring devices to actually measure the true weight of a single object in various places such as Death Valley, sea level, and mountainous areas such as the Alps and even the Himalayas. The weighing process was not in a vacuum, but in an atmosphere normal pressure. The study showed a continuous decrease in weight of the object as it was carried to a higher and higher altitude. Since it was a ambient atmospheric pressure, the buoyancy of the object was taken into account by the measuring process.

 

[theduckguru]

Why would throwing things in or taking things out alter the level of a swimming pool? Granted the water level in the pool may change, but the level of the basin remains constant. The question wasn't what happens to the level of the water inside the swimming pool, the question was what happens to the level of the swimming pool.

 

[windapp]

Why would throwing things in or taking things out alter the level of a swimming pool? Granted the water level in the pool may change, but the level of the basin remains constant. The question wasn't what happens to the level of the water inside the swimming pool, the question was what happens to the level of the swimming pool.

I'm pretty sure he meant the level of the water. Somebody is being a Smart*&*.

 

[PfcAJ]

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology - Carl Sagan

 

[windapp]

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology - Carl Sagan

True, but I wouldn't know how to farm, and I am still dependant on farmers for all my food.