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Hi, I am a teacher for the Algebra Academy Summer Program. Our project this year is to build and launch hot air balloons. We try to apply some basic math and physics to our projects. I found the following information ZOnline: Given a helium filled balloon surrounded by the atmosphere (air), gravity will pull harder on the air than the helium filled balloon. This more dense air pushes the helium filled balloon up. I agree that gravity is pulling on both the helium and air, but I have a hard time understanding how gravity can pull harder on one thing compared to another thing. I appreciate how the on line person explained why the helium balloon goes up, but I need to know/ be able to explain to myself and ultimately my students what this means (gravity pulling harder on one thing compared to another thing). Thank you. Johnny, Algebra Academy.
Question Date: 2018-05-05
Answer 1:

This topic is weird to think about, so I completely get where you're coming from. Hopefully I can do my best to help everyone understand.

To verify what you read, the gravitational attractive force between the earth and air atoms is greater than with earth and helium atoms (as long as they are the same distance away from the center of earth.) Gravitational force between two objects (be it the earth and air, earth and helium, or any other two objects) depends on masses of both objects, as well as the distance between those objects. This is the equation (I know, bare with me) to describe gravitational forces:

F = G * m1 * m2 / r2

F = gravitational attraction
G = gravity constant
m1 = mass of object 1
m2 = mass of object 2
r = distance between the two objects

This equation puts into context how gravity works, and why masses of heavier objects like those in air (oxygen, nitrogen, argon) will have a stronger gravitational force exerted upon them by the earth when compared to helium.

However, like you may also understand, helium rises in air also because of the buoyancy effect. All liquids and gases in the presence of gravity exert an upward force—called buoyancy—on any object immersed in them. If the object is less dense (mass per volume) than the liquid or gas, buoyancy will make it float. For example, a cork floats in water because it is less dense than a cork-size volume of water. But it won’t float in air because it is denser than the same volume of air.

Both gravitational attraction, as well as the effects of buoyancy, are important when considering why a helium balloon floats. If you stress why gravity works, as well as why something can float (based on density), that should be a good way to get the message across to your students.

Hopefully this helps.

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