why cant a flying object just leave the atmosphere at a slower speed? why does it need to achieve ‘escape velocity’? if a rocket goes straight up at 100kmph without stopping, it should escape the atmosphere eventually right?
In: Physics
What gravity is is you constantly falling towards the centre of mass of, well technically everything in the universe simultaneously, but mostly Earth. I assume you’re sitting still right now. That means then that you are just falling towards the earth in a straight line and being rudely interrupted by a chair. That chair is falling towards Earth too, and being prevented from falling further by the ground. Now, stand up and run 100 meters in a straight line, or if you don’t want to do that, imagine what it would be like if you had. If gravity didn’t exist and earth wasn’t moving, and you could run on air, you would end that run floating an imperceptible distance above the ground, because the earth has curved away from your straight line. However, you are not floating right now. Why? Because of gravity. Or because you didn’t bother doing it. You’re propelling yourself forwards, but gravity is also adding some downwards movement to you, so whenever you get the opportunity to fall back towards earth, you will.
The next thing I want you to do is get up again and go to space. Or, if you can’t be bothered, imagine what it would be like if you had, but know that I’m judging you for half-arsing it. Imagine that you have magic shoes that let you run on the top of the atmosphere, but don’t prevent you falling through it. If you stand still on the atmosphere, you’ll fall directly towards earth in a straight line. But what if you run? You still have that downwards movement from gravity, but you also have sideways movement. If you can run 100 meters in one second and gravity can pull you down 100 meters in one second, then after on second, you haven’t moved a total of 100 meters directly towards Earth, you’ve instead effectively moved down a diagonal line, because you’ve both run 100 meters across and fallen 100 meters down. If you keep running and keep falling, the diagonal line you’re moving on will eventually collide with the Earth, but it’ll do so a long way away from the spot directly below where you started, and because the earth has been curving away from you, the total distance you fell will be slightly higher than the distance you’d have fallen if you stood still.
In fact, here, have a [diagram](https://imgur.com/Xm48MWn). The big circle marked Earth is Earth. The red dot is where you started. The horizontal grey line indicates you moving in a flat line. If there was no gravity, the end of that line is where you’d end up. The line stretching from there towards the earth shows the downward movement of gravity. If you stood still there, you’d fall down that straight line and hit Earth. However, when you do both at the same time, you follow the path of the green line. It’s curved because gravity is actually an acceleration, not just a movement – you fall faster and faster as you go down, so the curve starts shallow and gets steeper towards the end of the fall. And notice that when you hit the ground as the orange dot, you’re further “down” than the ground directly beneath your starting point, and the angle gravity is pulling you at has changed.
Taking that then, imagine what would happen if you could run so far in the time it takes for gravity to pull you down that the curve gravity pulls you down actually misses Earth completely. You wouldn’t keep falling towards the bottom of the image forever, because the angle gravity is pulling you down at is constantly changing. Instead, you’ll circle around the earth, and if you keep running fast enough to always miss the earth, you’ll eventually end up back where you started. Congratulations, now instead of gravity making you hit the earth, you’re in an orbit! The moment you stop running though, your speed will fall, and the curve gravity pulls you down will make you hit the ground again.
This should illustrate why speed is important then. To escape the pull of gravity, you need to travel sideways fast enough that by the time gravity has pulled you down to the ground level of where you started, the earth has curved away from you enough that you’re not only still in the air, but higher up in the air compared to the new ground than you were when you started. You’re always falling when you’re in gravity, but sometimes you can run so fast that you miss the ground and just keep falling and missing and changing direction and falling again and missing again forever.
Your theoretical rocket would certainly leave the atmosphere eventually, yes–but what then? Things up in space are still affected by Earth’s gravity almost as much as at the surface (at the level where the ISS orbits the gravity is around 93% of ground level), so as soon as you turn your rocket motor off, you’ll start to fall back to the ground. In order to stay in space you need to go sideways really, really fast, which is why you’ll see rockets start to tilt over almost immediately they’ve been launched–just getting the 100km or so to space is the easy part, the hard part is getting to the 18,000mph needed to stay in orbit.
You can escape the atmosphere at any speed. Escape velocity is the speed at which, under the resistance of gravity alone, you will move faster away from the object than the object can pull you in, due to gravity weakening with distance.
Just under escape velocity, you might go out very very far but eventually turn around and go back.
At or above escape velocity, the rate at which the object pulls you back in will drop to almost nothing while you are still moving away from it. You have escaped the body.
It is important that escape velocity does not account for atmospheric drag, and also changes depending on your distance from the object. Escape velocity from earth in low earth orbit is much higher than at the moon.
You don’t even need to reach earth escape velocity until you want to go to a different planet. But you need to reach moon escape velocity to leave the moon and go to earth
The “escape velocity” is the velocity needed to escape the gravitational field, which is still strong even after the end of the atmosphere layer, it actually goes infinitely far. And yes, you can go slower, this is the concept behind the idea of the space elevator.
To escape a gravitational field you need an amount of energy that depends on how deep in the field you are. In theory, it doesn’t depend on the path you take or the initial speed. That energy, expressed as kinetic energy, corresponds to a speed (the masses cancel out).
When talking about rockets, this means that a rocket engine must contain at least enough energy to reach that speed if it were in free space, otherwise it wouldn’t be able to escape earth. That energy can be released quickly, like in an actual rocket, or slowly, like in a space elevator.
Since atmospheric drag and rocket technology plays a role here, it’s more efficient to quickly release most of the energy at the beginning instead of going up slowly.
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