Why balloon float

However, since there is also a gravitational interaction pulling the balls down, there are more of them closer to the ground. This is why the density of air decreases as you move vertically upward. The density of air near the ground is about 1. But even over a distance from the bottom of a balloon to the top, the density of air changes—just a little bit. Now let's put an object in the air. I'm going to use a brick. I like the brick since it clearly doesn't float in the air, but also it has flat surfaces to make my explanation easier.

Since the tiny air balls are moving around, some of them are going to collide with the surface of the brick. When a ball bounces off the brick, it gives a tiny little push on that brick. The total force on one surface of the brick depends on the area of this brick and the pressure of the air. Just a reminder, the relationship between force and pressure can be expressed as the following equation, where P is the pressure, A is the area, and F is the force.

So, if you have a large surface area and a small pressure, you can still get a large force. Because there are more gas balls closer to the ground, the pressure depends on the density of air, and, remember, the density depends on the altitude.

This means that the force from the air pushing on the top of the brick is different than the force on the bottom of the brick.

It's best to describe these collisions in terms of pressure and model the change in pressure with the following equation. So as y increases, the pressure decreases.

Note: This linear relationship is only approximately true. When you get really far above the surface of the earth, it doesn't work. But with this, you can see that the force from the air on the top of the brick should be less than the force on the bottom of the brick.

Notice that the forces pushing on the left and right sides of the brick are at the same height. This means that the net force in the horizontal direction would be zero—they cancel. But the force pushing UP on the brick from the bottom is greater than the force pushing DOWN since the bottom of the brick is at a lower altitude—even by just a little bit.

If the brick has a height h , then the total force from the air in the vertical direction would be:. Notice that I skipped some algebraic steps, but it's not too difficult to see how that works out.

But wait! If I multiply the height of the brick h by the area of the bottom A , I get the volume V of the brick. When you multiply that mass and the gravitational field g , you get the weight of the air displaced by the brick.

This is the famous Archimedes' principle. It says that when an object is in water, there is an upward buoyancy force on the object. The value of this buoyancy force is equal to the weight of the water displaced. But it also works for displaced air.

Yes, there is an upward buoyancy force on the brick. The brick doesn't float like a balloon because there is also a downward gravitational force on the brick—and this downward force is much greater than the upward buoyancy. Oh, here is the cool part. It doesn't even matter if you replace the rectangular brick with a spherical balloon. The buoyancy force still just depends on the density of the air and the volume of the object.

A birthday balloon and helium together weigh less than an equal volume of air, so the balloon rises. If you add weight to it, the balloon sinks.

If you add only enough to balance the force of buoyancy, the balloon will float in mid-air. Forces of Flight On This Page. What Is Buoyancy? Just a tad about how bouyancy works. Gravity acting on such a fluid creates this pressure, which is referred to a hydrostatic pressure. To make a long story short, the external pressure of the air is greater at the bottom of your ballon than at the top.

So the net external pressure on the ballon is imbalanced, providing a net upward force equal to the weight of the displaced air. If you put your ballon in space I'll ignore the observation it would pop , there is no pressure, and therefore no imbalance of pressure to create the bouyancy effect. Sign up to join this community. The best answers are voted up and rise to the top.

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