Date: Thu, 24 Oct 2002 10:16:11 -0600

Author: Jerry DiMarco

Subject: Re: He balloon rising


Here is an actual example to work with:
It is an altitude profile from a helium balloon experiment conducted by
students recently. It looks fairly linear to me. The answer to your
question is either simpler than we think or it is the case that some of the
variables cancel each other out.
The students are participating in a high altitude balloon flight
program set up to give them the opportunity to design and conduct
experiments and research projects. I became involved when they started
borrowing equipment to test out their ideas. The high altitude photos have
kept me interested. It is amazing how quickly (< 100,000 ft) the sky
becomes black. Makes you realize how thin our atmosphere really is...


At 05:47 AM10/17/2002, you wrote:
>A student asked me this the other day: If you let a He-filled balloon go,
>it will rise. Will it rise at a steady rate (i.e., have an upward terminal
>speed -- similar to a fealther falling downward)? I said probably not, and
>there are many variables to deal with. As you go higher, gravity weakens,
>air pressure decreases, and air temperature (in most parts) decreases in
>our atmosphere. The student got me thinking about all of this. Would the
>balloon have a constant upward speed near Earth's surface (i.e., assuming
>gravity, pressure, and temperature were constant)?


Jerry DiMarco
Manager of Lecture Demonstrations and Instructional Labs
Montana State Univ., Physics Dept.
Bozeman, MT

Our Motto: "We don't use anything the way it was meant to be used."