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?

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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

14 Answers

Anonymous 0 Comments

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Anonymous 0 Comments

Yes. The key part of your question is “without stopping.” That implies an absolute immense amount
of fuel, more than a rocket can possibly hold.

Things can’t just go at a set speed in atmosphere without constantly pumping in energy to fight drag and gravity.

Anonymous 0 Comments

Escape velocity is what is needed for an object that goes up to keep going up.

The problem with a rocket going straight up at 100 kph and just maintaining that is that you will end up running out of fuel before the earth runs out of gravity. If you were to create a better, more efficient propulsion system then you would eventually get far enough away at just 100 kph you would no longer fall back down and continue outwards.

So its not that it can’t work, its just that we don’t have the technology or capability at this point to make it work.

Remember, gravity is supplying a constant acceleration down towards earth. The farther you get from earth the lower that acceleration is.

Escape velocity is a concept that basically says “If I start at this point going at speed x straight up, I will move so far away from the planet that gravity will never reduce my outward velocity to 0.”

Most of our rocket launches aren’t concerned with reaching an escape velocity, they want orbital velocity. We don’t send many things away compared to what we want to stay nearby.

Anonymous 0 Comments

If I throw a ball into the air, the gravity of planet Earth will pull it back down and the ball will fall. If I throw it a little stronger, the ball will be in the air longer, but still will eventually fall back to Earth. I would have to throw the ball really really really strongly to make it go so high that Earth’s gravity doesn’t pull it back down.

That’s sort of the same idea here. A rocket that isn’t going very fast will also be pulled back down, and won’t be able to escape Earth’s gravitational pull. It has to go really fast or else it will fall back down, even if it doesn’t happen immediately.

Anonymous 0 Comments

You’re right, escape velocity is just a term for the initial speed something would need to escape earth’s gravitational pull to such an extend that it reaches an orbit. But we don’t just toss things into space, as that would result in a ridiculous force on the spacecraft and the people in it. Keep in mind that the escape velocity gets lower as you go higher.

Anonymous 0 Comments

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).

Anonymous 0 Comments

Yea, but getting into orbit isn’t about just getting out of the atmosphere, it’s about going sideways really fast. If you went straight up at 100km/h you’d be in space in about an hour but you’d fall right back down as soon as you turned off your engines.

Anonymous 0 Comments

Suppose you’re in a spaceship near to a planet. Let’s assume for a moment that the planet is very very tiny so we’re never at risk of crashing into it, and that it has no atmosphere. Let’s also assume that our rocket has run out of fuel. If our rocket’s initial speed is low enough, we will remain in orbit around the planet – specifically we will trace out an ellipse around it. If the speed is high enough, we will keep getting further and further from the planet – specifically we will trace out the shape of a hyperbola. Somewhere in the middle there is a speed, depending only on our initial distance from the planet (EDIT: and the mass of the planet), which marks the boundary between those two outcomes. This is the escape velocity.

Now in real life there are a few complications. Depending on the direction we’re travelling, we might collide with the planet. This is possible regardless of how fast we’re going. If we’re close enough to the planet that its atmosphere causes substantial drag, then things get a lot more complicated and whether or not we can escape its gravity depends on the density and viscosity of the atmosphere, the path we take through it, and how streamlined our rocket is. And if we still have fuel left and are using the engine to accelerate, this also complicates things. So “escape velocity” is not really a very relevant concept when we’re talking about a rocket that is on the ground on a planet with a thick atmosphere.

But if we’re already far out in space with no atmosphere around us, we’re orbiting a body and we want to leave it and travel to a different body, it’s a very useful concept. The “cost” of maneuvers out in space is often measured in terms of “delta v”, i.e. the amount by which our engine needs to change our speed. The difference between our current speed and the escape velocity is the delta v we need to escape the orbit of the body.

Anonymous 0 Comments

Think of gravity like a rubber band. Not taking in frivction or resistance of any kind, the rubber band will continue pulling it back correct? So the idea is to go as fast as possible t break free of the rubber band which is a constant force (gravity) pulling back on the object. Less energy/fuel is used to get away from earth by going faster. Once it’s in orbit it’s actually the inertia of the object fighting the force of gravity trying to pull it back in. Once the object slows down it will fall back to earth. If it keeps going out of gravity’s pull (massive amounts of energy) it will still be acted on in some way by other large masses

Anonymous 0 Comments

The atmosphere is very thin compared to the size of Earth. Low Earth orbit is just a few hundred kilometers up, meaning the strength of gravity remains pretty much unchanged.

If you go straight up you have to keep your rocket engines running all the time, even beyond the atmosphere. Sure, you can do it slowly. If you go sideways once you reach orbit you no longer need any propulsion to stay there.