Date: Fall 2013
I have a question. My mom has this rubber like exercise ball that when she fills it with air is pretty light (or so it feels like it) and bounces all around. I took the rubber cap off one day and put a 5 pound exercise dumbbell weight inside to see if it would still feel light. It did not go so well. I thought maybe if I blew it up, the air inside would lift the weight and it would still feel pretty light. I know that helium is used to make balloons float but I do not have that in the house. How do I find the right balance between the weight of what is in the ball, to the weight of what is outside the ball, so that it still bounces after putting a heavy item in it or still feels light? Did I not put enough air, do I have to use helium? What if I do not have that. Can I maybe put a water bag inside? Guess I was thinking more about how heavy things are able to float but that is against water how do we do this against air?
Thanks for the question. I applaud your mother for exercising and I applaud you for your curiosity. The question of whether something float or not depends on how its density compares to the density of the medium. For instance, a ship floats because it is overall less dense than water. Of course, a piece of steel used to make the ship will sink because steel is more dense than water. But you really have to consider all of the air in the various parts of the ship. A helium balloon will float in air because it is overall less dense than the air around it. If you take high school physics, your teacher will derive an equation that will tell you whether or not something floats. You can look up "buoyant force" on the Internet for more information.
I hope this helps.
It all comes down to density and dispersion.
Density is mass to volume. If something weighs more in a small package, high density. It something is light but takes up a lot of space, low density.
A pound of rock is about a handful. High density. A fluffy pillow may weigh one pound, but it will envelop our whole head! Low density.
Gasses do the same thing.
Also involved is the dispersive nature of gases. Gasses will disperse evenly over the entire volume of the container(our atmosphere!). For dispersion, think of adding a single drop of dark coffee to a whole glass of clear water - at first, you see the coffee drop, then it disperses so you may no longer see it.
So, you must contain the gas of lower density in a bag in order to lift the bag. This effect is quite weak for gasses - and the gas leaks out of the bag!
Hope this helps! Peter E. Hughes, Ph.D. Milford, NH
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