This is one of the coolest videos I’ve seen in a while: during a routine reboost of the International Space Station to a higher orbit, the astronauts on board show that the station tries to leave them behind!
What a fantastic example of Newtons’s First law: an object in motion tends to stay in motion unless acted upon by an outside force. As the ISS circles the Earth, all the forces on it are balanced. You can think of it this way: the force of gravity pulling it toward the Earth is balanced by the centrifugal force (or the centripetal acceleration, which is equivalent*) outward. Because there are no leftover forces on the ISS, it feels like it’s in free fall, what some people call weightlessness. No force means no acceleration which means no weight.
However, that’s not always the case. Even a few hundred kilometers up, there’s air. It’s thin, but over time it robs energy from the ISS, dropping it lower in its orbit. This is called drag, and it’s a very tiny force (too small to feel on board the ISS), but it adds up over time. To prevent the station from falling too far and burning up, every now and again low thrust rockets are used to push it up into a higher orbit.
But that applies a force that is not balanced! Read More