Helium lift (Part 2)?

Question

If a balloon filled with Helium is released on a windless day, what is the rate at which it will rise? Does it keep ascending at a constant speed or does it rise faster and faster as it goes up?

Answer 1

I would say and assuming that the volume of the balloon keeps on expanding it will rise slower and slower as the density up above is continually decreasing. If the volume remains fixed it will reach a particular air density level and will remain there.

Let's do it slow.
The balloon of mass m starts rising with acceleration a as the buoyant force Fb is acting on it. We should not forget that a force of gravity W is acting on it as well.
The total force is then
F(total)= Fb- W
also
F(total )=m a
m= p(Helium) + mass of the balloon
a is acceleration with which the balloon ascends. There are other forces at work too. Since it moves upward the balloon has to move air out of its way and that is air resistance or another force Fa.
Now
F(total)= Fb- W - Fa

As the balloon rises up in enters regions of lower pressure and therefore lower densities. It has to expand or it has to blow up or remain suspended that means remain at the same level since
F(total)= 0 since Fb=W

If it flows up... well it was a good balloon and story ends here.

If it expands it continues its ascent since it has increased in volume and that means it will displace more air and gain more buoyancy.

Just some trivia. Did you know that Joseph Kittinger was the first person ever to be in the upper Earth atmosphere was a person who got there in a helium filled balloon just before May 5, 1961 when Mercury astronaut Alan Shepard: did that?