Hot Air Balloon Descent: Does it Keep Accelerating Until Impact?

In summary, the conversation discusses whether a hot air balloon, cooling enough to start descending, will continue to accelerate until it hits the ground. It is assumed that the air inside the balloon does not cool anymore and the air pressure remains constant at all altitudes. There is a mention of testing the acceleration of a falling coin in a pool and the concept of terminal velocity. It is also mentioned that the balloon will become more buoyant as it descends due to an increase in atmospheric density. However, there is a complication with the atmosphere maintaining a temperature differential. The conversation also touches on the impact of air resistance on the acceleration of the balloon and the controls that can influence it. Finally, there is a mention of the Millikan experiment and its
  • #1
frankcm
3
0
If a hot air balloon cools enough to start descending, does it keep accelerating until it hits the ground? Assume that the air inside does not cool off anymore, and pretend that the air pressure stays the same at all altitudes. I tried testing it by dropping a coin into a pool. It didn't seem to accelerate but I think it might have reached terminal velocity from the water's "wind resistance".
 
Physics news on Phys.org
  • #2
I don't know the answer for it likely depends on the initial height and whether there is a function to influence the air resistance, e.g. a valve. However, there won't be an increasing acceleration. E.g. the Millikan experiment is based upon this.

You might want to read about the terminal velocity: https://en.wikipedia.org/wiki/Terminal_velocity
 
  • #3
All other things equal, the balloon will become more buoyant as it descends because atmospheric density goes up as altitude decreases. So it's possible the balloon will accelerate (in the form of slowing down).
 
  • #4
David Lewis said:
All other things equal, the balloon will become more buoyant as it descends because atmospheric density goes up as altitude decreases. So it's possible the balloon will accelerate (in the form of slowing down).
The balloon is under ambient pressure regardless. Its density increases as well. A simple comparison of balloon initial density versus atmospheric density as a function of altitude will not yield a correct answer here.

As the balloon descends and is compressed under increased ambient pressure, it does not maintain its initial temperature. It is compressed in an adiabatic fashion and heats up. One might be tempted to think that this will cause it to gain increased buoyancy. But there is a complication. All things being equal, the atmosphere maintains a temperature differential of its own -- the adiabatic lapse rate.

But all of that is irrelevant. Look back at the initial post. We are asked to consider an atmosphere with constant pressure at all altitudes.

If there is air resistance, then fresh_42 nailed the answer. If there were no air resistance (an unrealistic atmosphere with no viscosity) then indeed, the balloon would accelerate downward until finally crashing into the ground

fresh_42 said:
whether there is a function to influence the air resistance, e.g. a valve.
The controls in a hot air balloon are on the propane burner (heating) and on a vent at the top of the balloon (cooling). Neither of these has a significant effect on the huge air resistance of an inflated balloon.
 
  • Like
Likes sophiecentaur
  • #5
fresh_42 said:
I don't know the answer for it likely depends on the initial height and whether there is a function to influence the air resistance, e.g. a valve. However, there won't be an increasing acceleration. E.g. the Millikan experiment is based upon this.

You might want to read about the terminal velocity: https://en.wikipedia.org/wiki/Terminal_velocity

I'm not so sure... Millikan's drops were stable in mass and volume, but a descending hot-air balloon will probably lose volume and gain mass. That must result in a continually decreasing buoyancy. Hence, the speed will increase...
 

1. How does a hot air balloon descend?

A hot air balloon descends due to the laws of physics. As the air inside the balloon cools, it becomes less buoyant and begins to sink. The pilot can also control the descent by releasing hot air from the balloon's burner.

2. Is it safe to descend in a hot air balloon?

Hot air balloon descents are generally considered safe, as long as the pilot is experienced and follows safety protocols. However, it is important to always follow the pilot's instructions and listen to safety briefings before the flight.

3. How long does it take for a hot air balloon to descend?

The descent time for a hot air balloon can vary depending on factors such as wind speed and direction, weight of the balloon, and the pilot's technique. On average, it can take anywhere from 5-15 minutes to descend from a high altitude.

4. Can a hot air balloon make an emergency descent?

Yes, in case of an emergency, a hot air balloon can make a rapid descent. This is done by releasing hot air from the balloon's burner and using the deflation vent at the top of the balloon. The pilot is trained to handle emergency situations and ensure the safety of all passengers.

5. How do you control the landing of a hot air balloon?

The pilot can control the landing of a hot air balloon by adjusting the amount of hot air being released from the burner. By releasing more hot air, the balloon will rise, and by releasing less hot air, the balloon will descend. The pilot will also look for a suitable landing spot, taking into consideration wind speed and direction.

Similar threads

  • Introductory Physics Homework Help
Replies
7
Views
3K
  • Introductory Physics Homework Help
Replies
4
Views
4K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
7
Views
3K
  • Introductory Physics Homework Help
Replies
32
Views
3K
  • Classical Physics
Replies
6
Views
1K
  • Introductory Physics Homework Help
Replies
5
Views
8K
  • Introductory Physics Homework Help
Replies
4
Views
4K
  • Thermodynamics
Replies
1
Views
2K
  • Other Physics Topics
4
Replies
115
Views
28K
Back
Top