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
Yes, if you were going straight up at 100kph with a rocket that had the fuel to do it then you could keep going.
Escape velocity is the velocity you need for unpowered flight to go from the current position to infinity. At the Earth’s surface this is 11kps. Any slower and you’d eventually come back down (this doesn’t take into account getting caught in another body’s gravity such as gravitational interaction with say the Moon).
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.
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.
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
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