I am just pointing out that wind (gas) has high entropy. It is chaotic and turbulent by nature. It is just a basic piniciple that I think is relevant in this argument. A low entropy wind would be the exception not the rule.
Ah. You're stuck on the problem of the balloon matching the velocity of the wind. (I guess.)
So your concept is that a balloon could never match the velocity of the wind because gases have more entropy than a solid (latex). (I guess.) Or is it that solids have more mass than gases, and since [your words] "mass is a objects resistance to acceleration" a gas (wind) could never move a solid (latex) as fast as the wind is blowing?
The issue is kinetic energy, not entropy. Yes, gases flow over (around) a solid, but that causes a drag force. Wind is pretty good at transferring kinetic energy to a solid. I think!
The momentum of a body is the product of its mass and its velocity. Isn't it?
And yes, the wind has less mass, per volume, than a latex balloon. But doesn't time have something to do with it? Impulse is the term that quantifies the overall effect of a force acting over time. I think!
The relationship between a force and the time that it acts in to change the momentum of an object is given by the formula FΔ
t = Δ
p. Where F is the force that acts, Δ
t is the time for which the force acts, and Δ
p is the change in momentum. If time increases, momentum increases. I thought!
... a hot air balloon has a LOT of mass but can be naturally buoyant. It's resistance to acceleration far exceeds the air around it.
Meaning that the resistance of the balloon to acceleration far exceeds the ability of the the air around it to accelerate because the mass of the balloon is greater than the air around it? If the air isn't moving there's only going to be air molecules randomly colliding with the entire surface of the balloon. Air pressure, in other words.
But the wind is blowing. The wind is flowing around the balloon. New air is always coming in. That's a lot of mass over time. The mass of air that flows around the balloon over time is going to be far greater than the mass of the balloon. And since air flowing around an object transfers kinetic energy to the object the balloon is going to accelerate. The wind isn't going to stop. So the kinetic energy is going to keep coming. What's hard about that?
Are you picturing the air flowing over (around) the balloon with no transfer of energy at all? Why do balloons move with the wind at all? Or is it your belief that the balloon will only be nudged and stop accelerating because the energy transfer will be so low? Because of the mass per volume of solids versus gasses?
The ability of wind to do work is pretty well known. I mean how does a windmill generate electricity? How did HMS Victory sail? Didn't Victory out mass the air around her? The blades of a windmill out mass the air around them... plus the wind turns the generator... C'mon. Pushing a hot air balloon should be pretty easy.
Victory never sailed as fast as the wind she was in, before the wind or close-hauled, but there's reason for that. I'll get to it.
Straighten me out, Everyone. Returning to FΔ
t = Δ
p... Would F decrease as the balloon moves faster? The wind wouldn't be moving as fast relative to the surface of the balloon as it was earlier. But F wouldn't decrease to zero until the relative velocity of the balloon and the wind equal zero.
You do think that wind can accelerate a latex balloon to at least
some velocity? But
why would the acceleration stop before the velocity of the balloon matches the velocity of the wind? At
what point would the acceleration stop? At what percentage of the wind velocity?
Here's what I think in my artless way...
Over time the wind continues to do work. The balloon - and the gas inside it - gains momentum . There is a drag force on the balloon as long as it has a velocity different than the velocity of the wind, and there is a fluid flow over (around) the balloon.
When the balloon has a velocity lower than the velocity of the wind the drag force comes from the wind blowing over (around) the balloon, and the force is in the opposite direction to the relative motion of the balloon through the fluid flow; which is in the direction of the wind. I think you could call that thrust.
If the balloon were to have a velocity greater than the velocity of the wind, that would reverse the relative motion of balloon through the fluid flow. There's a drag force that also comes from air blowing over (around) the balloon, but the force is in the opposite direction to the wind. What is commonly called drag or wind resistance. The opposite drag forces will keep the velocity of the balloon steady. When the velocity of the balloon matches the velocity of the wind there is no drag force.
While the balloon is accelerating, there is a fluid flow around the balloon, and there may be turbulence. I'll keep it simple because turbulence is complex beyond my competence. Vortices may disturb the surface of the balloon. It may rustle. The balloon could spin or dance around. All that happens only when there's a mismatch between the balloon's velocity and the wind's velocity. That's what we see most often so that's what we would
always expect to see if we were using system 1 thinking instead of system 2 thinking. Slipping some psychology in.
Over time the velocity of the balloon increases to match the velocity of the wind because the wind continues to transfer kinetic energy into the balloon until F equals zero. The turbulence stops.
Once the balloon matches the velocity of the wind the balloon can't continue to accelerate, because at that time the drag force in the opposite direction to the wind would take over. The balloon is bumping into the air in front of it. It would take kinetic energy to overcome the drag force but there's no further source of kinetic energy. The wind can't push something faster than it itself is blowing.
And now there's a balance between drag forces in opposite directions. The balloon continues to travel along with the wind at the same velocity. The is no fluid flow past the balloon so no vortices, as there are in the flapping flag example mentioned earlier. So the surface of the balloon is still. There are no forces coming at it from a different direction. Nothing can accelerate any part of the balloon.
Square-rigged ships sailing before the wind can never sail as fast as the wind because there's another factor, which is drag from the water. But the wind does continue to transfer kinetic energy into the ship as long as it blows. It moves the ship across the Atlantic... in time. A lot of kinetic energy.
But if you think wind can't continue to accelerate a free floating hot air balloon until the velocity of the balloon and the velocity of the wind match... you'll have to show why the wind can't continue to transfer kinetic energy into the balloon before their velocities match. After all, the balloon accelerates in a wind even in your model. At what point, in your model, does the acceleration end?
Do you have an additional source of drag in your model" Such as the drag from the water in the ship example?
In my concept, when there is no fluid flow over (around) the balloon, the surface of balloon is static. And a bunch of balloons are static to one another. And balloon passengers feel no wind. A passenger could hold a the flimsiest half-inflated "mylar" balloon and there wouldn't be a rustle. Or a ribbon, or string, whatever. It's against common experience, but if someone can't get past that, that's a psychological problem.
If anyone has some comments or corrections I'm open. I only know basic physics.
I'll add some things. It's a classic problem in basic physics.
https://physics.stackexchange.com/q...-effect-of-constant-wind-on-a-hot-air-balloon
If a hot air balloon is being carried by a continuous wind current in a particular direction, in which direction will the flags on its basket wave?
The balloon will asymptotically approach the wind velocity. As it gets close, the force due to drag will decrease because the apparent wind will be small. To the extent it matches the wind velocity, flags will hang straight down. To the extent it is (barely) below wind velocity, the flags will point (just a little) in the direction of the wind. If the wind slows down a bit, the flags will point backwards until the balloon slows down to the new velocity.
answered Jan 16, 2016 at 15:26
Ross Millikan
- Since the wind is moving the balloon and given the size of hot air balloons can the speed of the balloon reach that of the wind?
– ispirato
Jan 18, 2016 at 7:00
@ispirato: I said it in the first sentence. The balloon speed will approach the air speed asymptotically. It is the same as if the balloon were magically started moving at the wind velocity in still air. It will slow down, appoaching zero speed asymptotically. The two situations are equivalent-the balloon starts with a certain velocity relative to the air and that velocity decays away.
– Ross Millikan
Jan 18, 2016 at 7:03
https://physics.stackexchange.com/questions/240162/hot-air-balloon-trajectory-predictions
Hey there Physics friends!
I've been struggling with what seems to be a very basic physics question. Let's say that I have wind vector for a given altitude and location, and I want to calculate the flight path of a hot air balloon.
So once the balloon hits that wind, would you calculate forward distance based on the wind speed (so would the balloon speed be exactly the average wind speed) or would you use the standard drag equation to calculate the force of the wind on the balloon to determine the distance traveled by the balloon in a given time period?
Feb 27, 2016 at 8:36
Qmechanic♦
Yes you would have to use the standard drag equation. Imagine the balloon is at rest (or moving with a different velocity from the wind) initially. As the balloon hits the wind it can't suddenly reach the same velocity as the balloon, as this would require an infinite acceleration (which is impossible). Instead the wind applies a force to the balloon. This force will increase the balloon's velocity until it does match that of the wind (drag force is proportional to the relative speed between the balloons), at which point the velocity of the balloon will match that of the wind. If you can assume that the time it takes for the balloon to reach the velocity of the wind is negligible (such as in the case of a large drag coefficient or cross sectional area of the balloon) then the velocity of the balloon could be assumed to be the same as that of the wind at all times.
answered Feb 27, 2016 at 7:18
Quantum spaghettification
