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
Everything humans ever built started off traveling in the same direction as Earth, and at the same speed.
It didn’t move away because while it was sitting on the launch pad, it was traveling 67,000 miles per hour too.
They changed the speed and direction, but not by much relative to what the Earth was already going at.
They never left Earth orbit.
Moon is on Earth orbit, remember. So on this large Earth-centered system of mass that orbits Sun, all Apollo 11 astronauts did was change their place within that system.
You will notice that the moon didn’t fly away at 67000 miles an hour either.
It’s like when you’r in a car that’s moving at 60 miles an hour and you throw a ball inside the car; the ball doesn’t immediately zoom backwards 60 miles an hour, because it is still carrying the momentum of the car’s motion. Relative to the car, you only threw the ball up and down. Unless some external force (like air resistance) causes some other force on the ball, the ball is going to go up and come right back down to your hand
There was no other force pushing Apollo 11, or any other space mission, in any other direction. It maintains it’s relative position to Earth, the moon, etc
It did move away from them. Otherwise they would have never been able to make it to the Moon. Now, if you’re asking why didn’t the Earth move away from them at a speed of 67k miles per hour, that’s because they are moving through space at a rate of (slightly) 67k miles per hour so keep up with the Earth.
Something that I don’t think the other answers have focused on enough is that things don’t just randomly slow down: there needs to be a force to change something’s speed. When you throw a ball out of a car, it stays moving forward at the same speed of the car *except* for the fact it is hitting otherwise-stationary air. By hitting the air, there is a force applied to the ball that slows it down.
In space, there is no air resistance, and no external forces that affect earth but not spacecraft. So, when on earth (or in earth’s atmosphere) you and everything around you is moving at that 67,000 MPH speed. When you leave the atmosphere, there is no mechanism to slow you down, so you are *still* travelling at that speed.
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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