Given that earth travels through space at a rate of 67,000 miles per hour, why didn’t it move away from the Apollo 11 astronauts when they left earth orbit?
In: Physics
Variations of this idea are one of the most common question on here, which I guess has to do with the unintuitive nature of relative motion, but we need to clear up this idea that motion is somehow tied to physical contact and that objects become “unbounded” or “decoupled” when they’re not in contact.
Let’s start with a basic primer. Let’s say you’re on an airplane going 500mph. Now let’s say you get up to go to the bathroom. Do you suddenly slam into the back of the plane at 500mph? Of course not, because you’re moving *with* the plane. Your motion relative to the plane is 0.
Now back to spacecraft. Where do they launch from? Earth. That means they’re orbiting the sun *with* the Earth. They started out already having Earth’s speed around the sun. (When you say Earth is moving through space at . 67,000mph, that’s is speed relative to the Sun. Speed and velocity aren’t meaningful unless you specify what what that motion is relative to.) So when spacecraft go to the moon, they’re still traveling *with* the Earth-Moon system.
Just to tie it back to my car example, let’s pretend there are 2 cars side by side in a 2 lane road, both going exactly 30mph (representing the Earth and the Moon). Now let’s say you want to throw a tennis ball from one car into the window of the other (the tennis ball is a spacecraft). When you throw the ball, does it suddenly go flying backwards at 30mph? No. If you aimed right, it goes directly into the other car’s window. The reason it doesn’t go flying backwards is because it was already moving with the cars to begin with, all you had to do was give it the velocity to get from one to the other.
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