Comment by layer8
4 days ago
Escape velocity is 25,020 mph (6.95 mps), so not completely surprising.
Note that escape velocity applies to a situation without continued propulsion and also without air resistance, but still you can imagine that the order of magnitude is similar.
Not surprising if you know that. Pretty surprising to me who didn’t.
Maybe you’ll like this too: The Earth’s speed around the sun is around 67,000 mph. So it moves significantly faster than the rocket, though not orders of magnitude. The solar system itself moves at 43,000 mph relative to its local neighborhood.
But speed is always just relative to some frame of reference. Acceleration, on the other hand, is absolute, and so might be the more interesting thing to look at here.
Acceleration is change in speed, so it is, by its very nature, relative just like speed is.
If I fall, I might accelerate at G meters per second, relative to the earth, but I don't absolutely accelerate. If the earth decelerates at the same time, I'm now both accelerating an decelerating. It's relative.
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Thank you for that precision "without continued propulsion", because when talking about rockets, physics teachers always talk about escape velocity, as if it was an absolutely necessary condition to escape earth's gravity.
But can't you escape gravity slower, just by going higher and higher at lower speed? Like a plane? (ie not vertically, but at an angle)
If you don't accelerate, my understanding is that you will slow down. In other words, it takes more and more energy to escape orbit. Eventually, if you don't accelerate, your speed drops to zero and you "fall back down". Escape velocity is about how much energy you put into your motion, not the velocity as such.
No, the absolute minimum speed IS required. Anything below that and you’re just making your orbit elliptical.
That seems wrong. If you have a way to maintain enough propulsion for long enough you can escape the gravity well at any arbitrarily low speed. You "just" need to maintain that speed long enough for the escape velocity from the gravity well to go below it as it diminishes with distance from the mass.
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Exactly. Your rocket can escape the earth at the speed of a slow elevator if you burn the engines continuously and you can carry an infinite amount of fuel and your fuel weighs nothing.
Since those constraints are impossible to meet in the real world, we have to get going fast enough to coast most of the trip on inertia after the fuel runs out.