Why do spacecraft and meteors heat up when they enter the atmosphere? People sometimes think it's because of friction between the atmosphere and the spacecraft, but the truth is a little more complicated.
You will need
A bicycle pump. Try to use a simple, hand-operated pump.
What to do
Using your thumb, block the hole where the air comes out of the pump.
Try to pump the pump, so you can feel the air inside it squashing.
Let the pump go back to its normal position.
Repeat steps 2 and 3 at least thirty times.
Feel near the end of the pump cylinder where the air comes out. If it feels normal to your fingers, try holding it against your top lip.
You should find it feels warm.
What's happening
You probably know that when you heat air up, it tries to expand and take up more room. It also works the other way around: when air (or any other gas) is compressed, it grows warmer.
Each time you squashed the air in the pump, it grew warmer. As it did, it warmed up the metal or plastic around it. When you released the pressure on the air, it cooled down again, but some of its heat was retained in the metal and plastic. After enough pumps, it becomes warm enough to feel.
Applications
When a spacecraft or meteor enters the atmosphere, they are going incredibly fast. When they hit the atmosphere, the air in front of them is compressed, so it heats up. This compression is what produces the heat of re-entry.
The effect works in reverse as well: when compressed gasses expand, they cool down. Many refrigerators and air-conditioners work on this principle.
Block the end of the pump with your thumb.
Pump it at least 30 times, with your thumb blocking the hole. I found it was easier if I braced it against my shoulder.
