# If terminal velocity exists why do things burn up when entering the atmosphere?

258 views

So, to my knowledge, things burn up when entering the atmosphere due to the friction caused by all of the air molecules hitting them on the way down which would make sense when an object gets faster, it hits more air molecules, and heats up, however, when an object reaches terminal velocity it no longer goes any faster, and objects that aren’t aerodynamic have a relatively low terminal velocity, meaning it may go just as fast being dropped from a tall building, and obviously, things don’t burn up (or even get hot) when being dropped from a tall building. So my question is why exactly being something falling into the atmosphere will burn up, but not if it’s dropped at a much lower height that would still reach it’s terminal velocity.

In: Physics

Because the atmosphere is thinner the higher up you go, objects falling back to Earth are able to gain a lot of speed before normal terminal velocity is achieved.

It’s like jumping off a bridge in to water. The air you travel through is thin but then you hit the much thicker water and it hurts. Reentry is like but with a gradual shift from thin to thick.

Because the atmosphere is so thin, things like parachutes wouldn’t work to effectively slow down, but if you were to release a parachute during the burn up phase it would be too much force and would destroy the parachute. You need to just take it with brute force until terminal velocity is reached.

Think water.

When you push down on water, it becomes compressed, right? Something similar happens with air. The compression from the pressure heats it up.

Meteorites are not, in any way, aerodynamic. If you dropped, say, a knife going point down, it wouldn’t burn up on entry (probably, haven’t tested it) because its so aerodynamic that it doesn’t ‘press’ down on air. But something larger that would, would heat up the air around it at such immense pressure that bam, you have friction.

Terminal velocity is based on the maximum velocity you reach due to gravity alone. You can go faster than terminal velocity if there are other forces acting on the object. This would include the momentum of an object already in motion.

things going through atmosphere heating up because of friction is a misconception. Friction contribute very little to heating. main heating comes from compressed air. As object comes hurdling through atmosphere, is usually goes so fast that air in front of it cant get out of the way fast enough. So its being compressed. And if you compress air it would increase its temperature. Also in your example of things dropping from building you dropping it with zero start velocity, objects entering atmosphere usually start with velocity in km/s range.

The ELI5 is that the item is entering the atmosphere well above terminal velocity.

Normally when we talk about terminal velocity we are talking about an item starting out at less then terminal velocity that picks up speed from gravity alone until air resistance exceeds the force of gravity

Already got some excelent answers, but one point that might be worth clarifying is that terminal velocity is really medium-dependant. The resistance from the stuff you are falling *through* (air in this case) is what counters the acceleration due to gravity. This is why something falling from orbit can be going fast enough to burn up.

Okay so the thing to know about terminal velocity is that it’s kind of a simplification. The faster you move through air, the more force you need to apply to push the air out of the way. As you fall, gravity will provide a force on you, and as this force accelerates you the air you’re falling through will start to provide more and more drag. At a point, the force of your weight pulling you down, and the force of drag pushing back at you will be equal. This is called terminal velocity.

But terminal velocity is only the speed you’d fall at if you were falling for long enough through air of a constant density. When something comes into the atmosphere from space. It was travelling in a near vacuum. So there was nothing to slow it down and depending on what it was doing out in space it could be approaching the earth at a speed of several kilometres per second. Obviously this is above terminal velocity, which means it will slowly start to slow down. But if something goes that fast there’s not really any way to slow it down much.

Some other important things to note are that the earth’s atmosphere is less dense higher up so terminal velocities will be higher. And also when something is going faster than the speed of sound, drag doesn’t really work the same way. At this kind of speed air can’t get pushed out of the way fast enough to let the object coming from space through. So instead it gets turned into a superheated plasma by the object coming through it. And it’s that, not friction, which makes objects burn up and produces the majority of the heat on the way down.

Terminal velocity is reached by two opposing forces in balance: gravity and air friction.
Air friction is proportional to air density and speed squared.
Air density reduces with altitude all the way ro zero.
There is no terminal velocity in space except the speed of light for totally different reasons.
So if you drop your object from high enough, it can achieve much higher speeds and as air density rises, friction does too and the terminal velocity decreases.
So, something that burns up in the atmosphere is coming from a place without terminal velocity into one that does, so must slow down, and any slowing down based on friction exchanges speed for heat.

The object enters the atmosphere at potentially hundreds of times terminal velocity…then the air tries to slow it to that terminal velocity…imagine the sheer force involved in slowing down a huge rock by thousands of kilometers a second

Well things can fall a lot faster without atmosphere, like in space.

If things go fast they heat when rubbing into other things, regardless if its air, or you swipiing your hand very fast on the couch.
As you said things have a terminal velocity aka. maximum speed when falling in air, this speed tends to be lower, than what needed to heat the thing up enough to make it catch fire.

&#x200B;

Except if its falling in space – and outside the air – it can accelerate to much higher speeds, and when it crashes into the air, it will heat up enough to burn up.

Both dropped things and things falling into the atmoshere reach terminal velocity, the difference is that dropped things start from zero, and their speed goes up, until it reaches terminal velocity. Things falling into the atmosphere typically go much faster than their terminal velocity, and need to *slow down* in order to reach it. And things that go fast have a lot of kinetic energy, which will get converted to heat as they slow down to their terminal velocity.

It’s because objects re-entering the atmosphere from orbit are already going much, much faster than terminal velocity.

If an object was somehow completely stationary relative to the earth and started to fall into the atmosphere, then it would not get that fast, and it would not burn up. Objects in low earth orbit are going at around 17,000 mph while they are in the vacuum of space.

So why don’t they just burn their rockets to slow down, and then enter the atmosphere? Because the amount of full needed would be huge – roughly the same amount that was needed to send it up there in the first place. So they use the air to slow down.

If your car is going 60mph and you hit a block of jell-o in the road your car is still going to crash before going through it.

An object entering the atmosphere is going faster than terminal velocity and the atmosphere is trying to slow it down to that speed, the friction between the air and the object is what causes the burning.