Helium ballon and hot-air ballon!!!

[peacefulheart]

1.A helium balloon will rise into the atmosphere until:


A. The temperature of the helium inside the balloon is
equal to the temperature of the air outside the balloon.
B. The mass of the helium inside the balloon is equal
to the mass of the air outside the balloon.
C. The weight of the balloon is equal to the force of the
upward air current.
D. The density of the helium in the balloon is equal to
the density of the air surrounding the balloon.

2. As a hot air balloon ascends, what is observed ?

A the external pressure decreases.
B. The mass of the hot-air balloon increase
C. The center of mass for the hot-air balloon system shifts to a higher point.
D. The surface area of the balloon decreases.

1. The answer for the first question is D and A for the second one.

2. I do not understand why C is not right if D is right for the first question. The helium balloon will stop rising when the upward buoyant force is equal to the weight of balloon. And then, from here, I got answer D by formula.

3.For D in the first question, how the two densities become the same. I understand that air density decreases when going upward. Is that any other way we can change the density of helium.

4. For the second question, the only way to get the right answer is that air density decreases and air pressure decreases when going upward.

5. In reality, if we assume that air density decreases are not significant since air density itself is 1.2 Kg/m^3 and variations will be in limits of 0.01 to 0.1, how can we change the speed of balloon. Do we need change the density of gas inside of balloon.

6.It is kind of long. Thank you very much for the patience .



thanks a lot.

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[image187]

1.A helium balloon will rise into the atmosphere until:


A. The temperature of the helium inside the balloon is
equal to the temperature of the air outside the balloon.
B. The mass of the helium inside the balloon is equal
to the mass of the air outside the balloon.
C. The weight of the balloon is equal to the force of the
upward air current.
D. The density of the helium in the balloon is equal to
the density of the air surrounding the balloon.

2. As a hot air balloon ascends, what is observed ?

A the external pressure decreases.
B. The mass of the hot-air balloon increase
C. The center of mass for the hot-air balloon system shifts to a higher point.
D. The surface area of the balloon decreases.

1. The answer for the first question is D and A for the second one.

2. I do not understand why C is not right if D is right for the first question. The helium balloon will stop rising when the upward buoyant force is equal to the weight of balloon. And then, from here, I got answer D by formula.

3.For D in the first question, how the two densities become the same. I understand that air density decreases when going upward. Is that any other way we can change the density of helium.

4. For the second question, the only way to get the right answer is that air density decreases and air pressure decreases when going upward.

5. In reality, if we assume that air density decreases are not significant since air density itself is 1.2 Kg/m^3 and variations will be in limits of 0.01 to 0.1, how can we change the speed of balloon. Do we need change the density of gas inside of balloon.

6.It is kind of long. Thank you very much for the patience .



thanks a lot.

your post is sort of confusing but I'll try to answer it simply. The balloon rises because it is less dense than the air, and it will stop rising when it arrives at a place in where the surrounding environment has the same density (atmospheric density decreases as we move up). The mass of the balloon doesn't change and since acceleration of gravity is the same neither does it's weight. Air currents seem pretty irrelevant to the question.

The hot air balloon rises because a flame heats the air, which causes
inc kinetic energy/widely spaced gas molecules, decreased density and then this acts like a helium balloon. The mass never changes. The center of mass doesn't either because the balloon would be expanding in all directions equally (inc surface area). The only logical response is A because air pressure decreases as we go up. And changes in density and pressure ARE significant... ever try to dive a few feet into a pool or drive a few feet up a mountain without popping you ears?? ouch!

 

[peacefulheart]

your post is sort of confusing but I'll try to answer it simply. The balloon rises because it is less dense than the air, and it will stop rising when it arrives at a place in where the surrounding environment has the same density (atmospheric density decreases as we move up). The mass of the balloon doesn't change and since acceleration of gravity is the same neither does it's weight. Air currents seem pretty irrelevant to the question.

The hot air balloon rises because a flame heats the air, which causes
inc kinetic energy/widely spaced gas molecules, decreased density and then this acts like a helium balloon. The mass never changes. The center of mass doesn't either because the balloon would be expanding in all directions equally (inc surface area). The only logical response is A because air pressure decreases as we go up. And changes in density and pressure ARE significant... ever try to dive a few feet into a pool or drive a few feet up a mountain without popping you ears?? ouch!

1. Thanks a lot for the explanation.

2. Could you please explain why C is not right for the first question? is not that true that the balloon stops rising when buoyant force of air is equal to the weight of balloon?


thanks

 

[image187]

1. Thanks a lot for the explanation.

2. Could you please explain why C is not right for the first question? is not that true that the balloon stops rising when buoyant force of air is equal to the weight of balloon?


thanks

no problemo

hmmm... your point makes sense but I don't see why thee would be an air current in the question. I've also never seen the word "buoyancy" used in anything other than talking about fluids and submerging things in liquids. As something with a constant weight sinks into water, the buoyant force increases with depth of the object until some equilibrium is reached and the thing floats. but in this question, an air current isn't really the same thing, or at least this is my idea. I feel like te air current would either be constant or erratic in some way. Why would there be an upwards wind with a force on the balloon that decreased continuously until the force was equal to the balloons weight at which point the balloon would stop and float in mid air, maintained by this constant wind gust? it doesn't really make any sense to me

 

[peacefulheart]

no problemo

hmmm... your point makes sense but I don't see why thee would be an air current in the question. I've also never seen the word "buoyancy" used in anything other than talking about fluids and submerging things in liquids. As something with a constant weight sinks into water, the buoyant force increases with depth of the object until some equilibrium is reached and the thing floats. but in this question, an air current isn't really the same thing, or at least this is my idea. I feel like te air current would either be constant or erratic in some way. Why would there be an upwards wind with a force on the balloon that decreased continuously until the force was equal to the balloons weight at which point the balloon would stop and float in mid air, maintained by this constant wind gust? it doesn't really make any sense to me

1. Thanks a lot for the explanation.

2. When the balloon stops rising, F(buoyant force of air.)=mg (weight of balloon including helium). From this equation, you can get the answer D. Air is a fluid.

3.If C is not right, just like you said, the force of the upward air current is not buoyant force at all.

thanks

 

[peacefulheart]

Could anybody make the reasoning clear for this question? thanks a lot .

 

[Deadlifts]

C. The weight of the balloon is equal to the force of the
upward air current.

You're assuming that upward air current = buoyant force. They aren't the same thing. A helium balloon released from the ground will rise regardless of the presence of a current. Draw your FBDs for the balloon and you'll see there's technically 3 forces acting on the balloon.

Also, http://forums.studentdoctor.net/archive/index.php/t-890579.html

 

[gabdolce]

I want to also echo the emphasis on the verbage "air current." This answer is literally wrong only because of the words they used. This answer is contingent on you being careful with very precise language - air current is not a bouyant force. They are completely separate entities: bouyant forces are intrinsic characteristics of all fluids while air current is a characteristic of the viscosity and consequently resistance of a particular fluid (air)