Here’s something that’s recently been puzzling me. Maybe some of you physics whizzes can help:
Let’s say I’m holding a helium baloon in my hand. I throw it toward the ground as hard as I can. Thus, it should have inertia that’s pushing it toward the ground. Air friction acts on the balloon so it doesn’t continue going down and doesn’t hit the floor (in my example). But what force pushes the balloon back up?
It’s called buoyancy.
Sorry for the short answer, but I like being first to respond. 
Here’s a better explaination:
So when you throw the helium balloon at ground, it has momentum toward the ground (but not a force) and its own weight. Working against its momentum are drag and buoyancy. Drag will simply slow it down. It wouldn’t impart a lifting effect. But the buoyancy, working in the direction directly opposite of gravity, not only slows it down, but then lifts it against gravity.
An air balloon would act similar to the point that its momentum would be drastically reduced by drag. But its gravity-supplied weight would still bring it to the ground.
It is the result of gravity. Gravity is pulling the balloon and the surronding air down. Air is denser than the balloon, so the downward force on the air is stronger than on the balloon.
It’s interesting - early plans for lighter-than-air objects called for a hollow ball to be emptied of air. Perhaps the plan was to use a pump or bellows of some kind. I saw a Renaissance-era drawing of an “air ship”, with a hull like a boat, and a single square sail, and four balloon-looking things which were to be filled with vacuum. Naturally, air pressure would have crushed any vacuum-filled object made with contemporary technology, but it was an interesting idea.
The thing is, as a kid I didn’t understand why this would work. Vacuum has zero mass, thought I, while helium and hydrogen have negative mass. Of course, I was totally wrong - vacuum would be better to fill your airship with than any gas, except that vacuum would not equal external air pressure. In order to keep your “vacuum bag” from imploding, you’d have to build it of such strong (and presumably heavy) materials that it would still probably be heavier than air, or at least heavier than a comparable volume of helium in a bag. Zut alors, no vacuum airships.
Relating to helium balloons and buoyancy…
Here’s an interesting experiment (that I must admit I’ve never actually tried in a controlled situation):
Drive along in car with the windows up (to avoid breezes that would screw everything up), with a helium-filled balloon or two floating free in the car. If you’re travelling in a straight line at a constant speed, they’ll settle against the ceiling someplace.
Brake moderately hard. You, the books sitting on the passenger seat, and most everything else will be propelled forwards in the vehicle. The helium-filled balloon, however, will move backwards.
Why? Within the frame of reference of the car (which is NOT inertial when you’re slowing down), the force caused by the decelleration is a body force, acting in exactly the same way as, say, a gravitational force, only sideways.
This counts for the air inside the car, as well. The lighter-than-air balloon will experience a “buoyant” force directing it to the rear of the vehicle. While clinging to the ceiling all along.
Like I said, I’ve never actually had occasion to try this. The explanation makes sense to me, although I fear my recounting of it may leave something to be desired. If I were willing to spend more time right now, I could probably make it much more rigorous.
All that air sloshing around in my car…
::giggle::
I’ve done this experiment. It’s pretty cool. You’ll probably also notice that as you go around, say, a sharp right turn, you’ll tend to lean toward the left, but the Helium balloon will lean toward the right. It’s the same reason as before.
Actually, I have experienced that.
In my van in winter, when the car has not entirely warmed up and I make a sharp, braking turn (e.g., inside a parking garage), the cold air from the back seat very definitely flows across my legs.
Actually, a balloon is an interesting way to learn about forces, because if you assume it is perfectly spherical (it isn’t, but it’s close enough) you can mathematically show why a balloon rises. It’s true that a balloon goes up because it’s contents are of much lower density that the surrounding medium, but there has to be a net force pushing the balloon up. The force s generated by the difference in pressure between the surrounding air at the bottom of the baklloon and that at the top – gravity creates a pressure gradient vertically. If you have a balloon in the space shuttle there’s no pressure gradient, so the balloon doesn’t go anywhere. You can write out the pressure ifferences and the vector directions and integrate these over the surface area of the balloon, and prove that you get the same result as if you assume simple buoyancy. Just another way science can make our lives more complex. But it does show the origin of the force that pushes the balloon up.