Gravity Vs Balloon

Classical Mechanics Level 1

It is well known that a balloon filled with helium flies upwards, against the force of gravity.

Why does a balloon fly against gravity?

Helium gas molecules are much smaller and lighter than oxygen and nitrogen molecules. The helium gas inside a balloon is under less pressure than atmospheric pressure. Gravity pushes upward on the balloon. The helium gas inside is less dense than the air around the balloon.

1 solution

Rohit Gupta
Feb 29, 2016

Firstly, the gravity acts on all the things that has mass, and it will pull the balloon as well in the downward direction. Still, the balloon goes upwards, against the gravity.

Next, the material of the balloon tries to compress the air inside it. Thus, the air inside the balloon is at greater pressure. But, this has nothing to do with balloon flying in the air. The inside pressure will create a force on the balloon rubber, throughout uniformly, and hence, the net force on the balloon due to inside pressure is zero. Moreover, for the motion of the balloon, a net external force is required.

For the balloon to fly against gravity, it needs a net force in the upward direction. The trick here is, the balloon is filled with a gas of lower molar mass. Gasses like helium and hydrogen can do this trick. The molar mass of air is nearly 29 grams and that of helium is 4 grams. When helium is filled in the balloon, then the air inside the balloon has less mass per unit volume and hence is of less density. Therefore, the balloon displaces the air more than its weight and the air buoyancy is more than the weight of the balloon. Therefore, it flies up against the gravity.

There is not the exactly correct answer in your choice set. Only a complex system of various interacting conditions can explain why balloons are flying upward. For example, fill by helium not a balloon but regular football ball, and the ball is not flying, though your "correct" condition is matched.

Vasiliy Znamenskiy - 5 years, 3 months ago

The answers are all poor. More collisions on bottom of balloon. Gravity creates a concentration gradient that leads to more dense air on the bottom surface. Temperature is relatively constant so there are more roughly equally energetic collisions on the bottom surface of the balloon. A net force results upward. The medium in the balloon results in a downward force. Helium is relatively light and a net upward force results.

Brian Space - 5 years, 3 months ago

@Brian Space , Hi, Thanks for the feedback, But I am little confused from your statements. Please elaborate on the following.
1) " More collisions on bottom of balloon " Collisions from inside the balloon or outside the balloon? And how do collisions relate to this situation?
2) " Gravity creates a concentration gradient that leads to more dense air on the bottom surface " What you said is true, but considering the size of the balloon the impact is not huge. One more thing even if the density is more at the bottom, still it will be lesser than that of the air. Therefore, the gradient of density has no role to play in here.
3) " Temperature is relatively constant so there are more roughly equally energetic collisions on the bottom surface of the balloon " Again I could not see any role of temperature here. Please elaborate your point of concern here.

According to Archimedes principle, If a body is less dense than the fluid and it is completely submerged in the fluid, then the buoyancy force is more than the weight. Here, because the air inside the balloon is lighter hence the air buoyancy is more than the weight of the balloon. Hence net force is upward and balloon goes against gravity.

Rohit Gupta - 5 years, 3 months ago

2 is important, the pressure gradient caused by gravity causes the balloon to float upward. Consider fluids without an external force. A helium balloon in pressured air chamber in space will stay where you put it in the absence of any other external forces. Accelerate the chamber and the balloon will travel in the direction of the acceleration as a pressure gradient is caused by the acceleration of the chamber.

Owen Berendes - 5 years, 3 months ago

@Owen Berendes – We are considering the balloon flying in the air, not in the vacuum. Are you considering air in the chamber or not?

Rohit Gupta - 5 years, 3 months ago

@Rohit Gupta – I am considering air in the chamber. with no air in the chamber (physics in a vacuum) balloons of any density will act exactly as Newtonian physics would predict they would "stay at rest" as the chamber moved about them or all fall at the same rate when in a sealed vacuum chamber on earth. Now put three balloons in the chamber, one with a density greater than the chamber gas, one with a density equal to the chamber gas, one with a density less than the chamber gas. They will all sit exactly where you put them since there is no pressure differential in the surrounding chamber. Now bring that sealed chamber to earth. The balloon which is more dense than the chamber air will fall to the floor, the balloon with density equal to the relative total density of air in the chamber will float at its equilibrium point between the floor and the ceiling, and the balloon of density less than the average density of the chamber will float to the ceiling.

Owen Berendes - 5 years, 3 months ago

@Owen Berendes – What you said is correct, I was thinking that you meant that the density gradient will occur inside the balloon. And in fact, density gradient should not be the exact term here, gravity is pulling both the balloon air and surrounding air, but if the density of surrounding air is more then it will eventually win the race. Thus the balloon goes up because the helium gas is less dense the air in the surrounding

Rohit Gupta - 5 years, 3 months ago

"What you said is true, but considering the size of the balloon the impact is not huge. " It is exactly the upward force that overcompensates for the downward gravitational force.

Brian Space - 5 years, 3 months ago

Given the fact that you always have the same number of molecules per volume/pressure/temperature, The weight of the molecules is the determining factor fool.

Simon Nightingale - 5 years, 3 months ago

No different numbers of molecules as a function of height thus the P gradient

Approx N=N_0 exp (-mgh/kT)

It's all collisions

Brian Space - 5 years, 3 months ago

Gravity Vs Balloon

1 solution

0 pending reports