Physics in Action: Buoyancy in Air

This lesson has not been reviewed.

Please purchase the lesson to review.

You've seen this many times in your life. You jump into a bathtub and the water level in the bathtub goes up. Imagine you were represented by this black weight. You jump into the bathtub, here represented by this red liquid, and the level of fluid goes up. When you go out, it goes down. You jump into the tub, it goes up. We say that you've displaced a certain amount of water, meaning that in order for you to get into the tub, a certain amount of water has to get out of your way. It has to be displaced. We can measure that by looking at the marks on this cylinder.
Now, why do we care about the water that you displace when you climb into the bathtub? Well, we care for one reason. A few thousand years ago, Archimedes told us that in fact, if you're underneath a fluid like this, you're buoyed up or supported by a force equal to the weight of the fluid--in this case, water--that you displace. That's why we care about the buoyant force. Now it's important to bear in mind that even if you're sitting at the bottom of the tub and you're not actually floating, you're still buoyed up by this force. That's why we care about the buoyant force and the water that's displaced.
Now let's look at this apparatus over here. This doesn't look like it's displacing anything but in fact, it is. Let's see what we have. We have a glass ball over here balanced by another smaller metal ball. Now, is this being buoyed up by some force? Well, think about it. This ball is, in fact, floating in a fluid. What is that fluid? That fluid is the air that we're breathing. You can't see it--in fact, you can't see the air level go up--but certainly this displaced a fluid, namely the air, just like this weight displaced the red fluid, and just like you will displace the water in the tub.
Well, if all that is true, what it means is that this ball is being buoyed up by a certain force, and its apparent weight is less than would be there if the air weren't there. Let's check that out with this demonstration. What we have now is this apparatus with a small ball weighted against this large glass sphere. What I'm going to do is put this inside this jar, called a bell jar. I will push it down on the seal. The way this is arranged is that this bell jar, this whole space in here, is connected by means of a pipe to a vacuum pump. When I close the valve over here and hit the switch, the pump will go chugga-chug-chug, and that's the noise it makes when it's sucking out all the air. Let's look at this carefully and see what happens.
Notice what has happened. As the air was pumped out, this ball sunk down, indicating that its apparent weight increased. Now its mass didn't increase, but what has happened is that the buoyant force helping it up, which contributed to its apparent weight before, has gone away. In fact, the ball now appears to be heavier. Its apparent weight has increased. This reflects the fact that the buoyant force, which was being supplied before by the air, is no longer there.
Let's see what happens if you let the air back in. First of all, the hissing sound is the sound of the air rushing back in. But notice, now that we've restored the air and returned it to atmospheric pressure, the balls balance exactly as they did before. This clearly demonstrates that any object in any fluid--in this case, air--is buoyed up by some force, even though you can't see the fluid and even though you can't see the fluid that you're displacing.
Fluids
Fluid Statics
Physics in Action: Buoyancy in Air Page [1 of 1]