Gravity and Buoyancy ```Name: Diane Status: student Age: N/A Location: N/A Country: N/A Date: N/A ``` Question: If gravity pulls on everything, why does a helium filled balloon float? Replies: Hi Diane A helium balloon will float for the same reason that a bubble will rise in water -- Just as the bubble is less dense than the water surrounding it, a helium balloon is less dense than the air surrounding the balloon. Because gravity acts on the air just like it acts on the balloon, the effect of gravity has no bearing if the balloon floats or not - it comes down to a matter of relative densities. Hope this helps. Bob Froehlich "Heavier" objects are pulled harder by gravity than "lighter" objects. Everything takes up space, too, though, so the ratio of weight(mass) to volume (known as density) determines what sinks and what rises. This concept is known as 'displacement' -- whatever space on object takes up has to displace (or move) something else out of its way to be there. When objects with different densities are in a fluid (such as air or water), the more dense objects sink lower than the less dense objects. The force of gravity on the object is strong enough to force the lighter object out of the way of the heavier object. Air is more dense than water, so water sinks below air. A rock is more dense than water, so it sinks in water and air. And helium, which is less dense than air, rises in air and in water. Even with the extra weight of the balloon, the helium is light enough to rise in air. Hope this helps! Burr Zimerman Diane, Gravity does pull on everything, but some things feel greater force than others. Floating in air is really the same action as floating in water. A material more dense than water will sink. A material less dense than water will float. A material less dense than water released from the bottom of a huge column of water will rise similar to how helium rises through the air. Helium is less dense than air. In outer space, helium will be pulled in toward the atmosphere by gravity. Dr. Ken Mellendorf Physics Instructor Illinois Central College Helium is less dense than air. Less dense things float on more dense things. Helium floats on air like wood floats on water. If you weighed one cup of water and one cup of wood you would find the wood weighing less! If you weighed a balloon full of air and the balloon full of helium, the balloon of helium would be lighter. Because we are surrounded by air, the helium floats. In fact, it is difficult to weigh helium because it floats off the scale! Robert Avakian Dear Diane, Yes, gravity does pull on everything. Since the atmosphere has mass gravity does indeed act on it. However, a helium balloon floating has not so much to do with gravity as it does with density. Density is determined by dividing an object's mass by its volume. Think of a bottle of Italian dressing, the kind you have to shake. It is mostly vinegar and oil with spices and herbs. The herbs are the most dense and sink to the bottom, then comes the vinegar with the oil forming a layer at the top. Even after you shake it, it will return to this state because items that are less dense will float in fluids that are more dense. Wood floats on water because it is less dense than the water. You float in the Great Salt Lake because you are less dense than the salty water. Now to the balloon. Helium has a density of 0.0001663 g/cm3. Plain old ordinary air has a density of 0.00120 g/cm3. The helium is less dense than the air and "floats" in the air. Carbon dioxide has a density of 0.00183 g/cm3. It is more dense than air and will "sink" in air. You can see this if you dissolve an Alka Seltzer tablet in a small amount of water. You can actually "pour" the gas over a lighted candle and extinguish the flame. It really works. I hope this makes sense to you. Write back if you want to know more. Martha Croll If there were no air, the helium balloon would sink. In the Earth's atmosphere, the Earth's gravity pulls downward on both the helium balloon and on the air surrounding it. Since the helium balloon has less mass than an equal volume of air, gravity pulls more strongly on the air. Thus the air will sink and the balloon will float, just like a cork will float on water. Richard Barrans, Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming Click here to return to the Physics Archives

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