Buoyant force definition

[SaintJude]

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Why is C wrong? Is there a difference b/w the buoyant force and the "force of the upward current" ?

 

[chiddler]

Even after a current, if it will continue to rise. If there is a current, this might be comparable to a force pushing the balloon upwards. There is nothing to stop it from moving upwards unless the current stops.

No, upward current is different from buoyant force. An upward current indicates there is movement of something pushing it up whereas this is not characteristic of buoyant force (placing a balloon of air in water will cause the balloon to rise spontaneously; you don't need a current).

Rereading what I wrote, i don't like my explanation 😛. Sorry i'm not great at physics. But I hope it helps a bit.

 

[milski]

View attachment 18610

Why is C wrong? Is there a difference b/w the buoyant force and the "force of the upward current" ?

Yes, there is a difference. Force of the upward current will be the force that you would get if the balloon was surrounded by raising air. The buoyant force is static - it's independent of the motion of the fluid.

While some of the raising of the balloon can be attributed to raising air, for a helium balloon that cannot be the only force acting on it. The buoyant force can easily be the single force acting on it - all you need is no updrafts. That makes D a better answer.

 

[MedPR]

Imagine blowing up a balloon and putting a hair dryer under it. It will rise due to the force of the air blown upward by the hair dryer, but it will continue to rise even more once it escapes (rises so high) the reach of the hair dryer force. The force making it continue to rise = buoyant force of the air.

 

[SaintJude]

Sorry to resurrect this post, but what I think I found slightly odd about Choice D is that it's suggesting that the density of the helium and air are supposedly changing.. until they meet an equilibrium?

Wait! Is this because density is actually a temperature-dependent constant and will thus vary as we move up in atmosphere..?

 

[milski]

Sorry to resurrect this post, but what I think I found slightly odd about Choice D is that it's suggesting that the density of the helium and air are supposedly changing.. until they meet an equilibrium?

Wait! Is this because density is actually a temperature-dependent constant and will thus vary as we move up in atmosphere..?

Density for a gas depends on pressure and temperature, so it certainly will be changing. Most balloon problems I've seen ignore the temperature difference since both gases are experiencing it plus it would require knowledge about atmospheric temperatures which is not trivial.

 

[chiddler]

air is thinner when you go up in atmosphere! same reason why climbers bring oxygen tanks when they ascend. becomes difficult to draw a full breath. as density of air decreases, the pressure on the helium balloon changes so its density in turn changes.