Magically Crushed Bottles

When landing in an airplane, a water bottle can seem to magically be crushed.

You arrive at the airport with your water bottle. It's one of those that has a silicone straw attached to it. As you are about to board the plane, you take a quick sip of water from the bottle and close it. You continue to board and the flight eventually takes off. However, a few hours in you feel that you're thirsty again and reach for your water bottle. This time, as you flip open the straw, all the water squirts out all over you. What went wrong? This article will describe the effects of air pressure and why this happened.

Air is all around us on the Earth's surface. It piles up for miles above us into the atmosphere. Just like how gravity pulls down on all the air in our atmosphere. The weight of all that air pushing down on us at sea level is about 14.7 pounds on per square inch. This may be hard to think about, but it can actually exert more force than 30 horses can. To read more about that check out the 30 Horses vs 1 Ball article. Moving back to the topic, you would expect that as you went higher up away from the Earth's surface, air pressure would become less because there would be less air being piled on. So as a rule, air pressure is lower at higher elevations. So, as you go up higher in an airplane, your air pressure will decrease also.

The technology in airplanes is a little more advanced though. If we really tried to breathe in the air at cruising altitude, we would need oxygen masks. To overcome this, airplanes have to pressurize the cabin so that you can breathe normally. To put numbers to it, air pressure on the ground is 14.7 PSI. In an airplane, it is pressurized at around 11 PSI when the real air pressure outside at cruising altitude is only 3 PSI. So now we've established the pressure difference inside the aircraft relative to the ground. Let's also establish that pressure will always exert a proportional force in all directions. So, if you open a water bottle and close it on the ground, the air pressure inside the water bottle is 14.7 PSI. This doesn't change when the airplane ascends because the water bottle stays closed, so the air can not enter or leave. Once in the air, there is now a situation where there is a water bottle with 14.7 PSI inside and the surroundings of only 11 PSI. This indicates that the inside of the water bottle is pushing out more than the air outside is pushing in on the water bottle. For this reason, when you open the water bottle at this altitude, the water is very likely to squirt out. This may be difficult to see with a normal plastic water bottle, but if you (and everyone around you) use a water bottle with a built-in plastic straw, you will definitely notice this. I have added a youtube example so you can see this in action.

Now we can finally talk about the title of this article. Magically Crushed Bottles. You may be starting to get a sense of why this happens. Let's transition to a normal plastic water bottle that has been emptied. If you open a water bottle when you are in an airplane at cruising altitude, it will pressurize to the surrounding cabin pressure. So we have an empty plastic water bottle sitting at 11 PSI. The plane begins its descent, and slowly the surrounding cabin pressure changes from 11 PSI to the 14.7 PSI that is normal at sea level. The water bottle ,however, always stays at only 11 PSI because it has been shut. The air outside of the bottle has higher pressure, so it pushes in on the water bottle which results in its apparent "magical" crumpling. I have added a youtube video that demonstrates this effect.