Air Pressure Demonstration ............................................

Air has weight, which causes air pressure. This simple demonstration should convince you that air pressure is very strong.

You'll need a jar, big enough to fit your hand inside. You'll also need a sandwich bag that's slightly bigger than the jar, and two or three elastics.

When we did this experiment we discovered that we also needed some duct tape. We'll explain why in a moment.

First, reach inside the plastic bag and try to pull it inside out. You will be able to do this easily.

It's easy to do because the air pressure on both sides of the bag bottom you are pulling up on is the same. Both sides of the bag are exposed to the outside air, so the air pressure on both sides is equal. The only force stopping you from pulling the bottom of the bag is the plastic material of the bag itself, which isn't very strong.

Next, put the bag inside the jar. Press down on the inside of the bag so that it completely fills the jar, touching the inside walls. Make sure the top of the bag overlaps the rim of the jar. Then wrap several elastics around the jar's rim and the top of the bag. You want to make an air-tight seal that won't leak when you try to pull on the bag; we had to wrap it tightly with duct tape to make a good tight seal. (We didn't show that here).

Now reach down into the bottom of the jar and grab the bottom of the bag. Try to pull it back up, outside the jar.

Amazingly, you will discover that it is impossible to pull the bag out. It will move a little, but you won't be able to pull it all the way out without ripping the bag or breaking the seal at the top. (That's why we needed the duct tape).

The reason it won't come out is because the air pressure on each side of the bag is no longer the same. The pressure inside where your hand is remains the same as before ... ordinary air pressure pressing downwards. But inside the jar, between the jar walls and the bag, there is hardly any air. The air pressure outside can't get in there, so as you pull on the bag, the air that's there just gets thinner and thinner. This partial vacuum has very little pressure ... nowhere near enough to balance the force of the air on the other side. The result is that there is a net force downwards from the pressure of the air outside, pressing down on the bag and preventing you from pulling it out.

This is just like what happens when you stick a toilet plunger or other suction-type device to the floor. Pressing it down forces most of the air out, so there is very little pressure inside. The full air pressure above the plunger pushes it to the floor and makes it hard to pull up.

The full air pressure from above (A) is much stronger than the air pressure inside (B) pushing up. So the net force downwards sticks the plunger to the floor.

Physics | Science & Math | Worsley School OnLine