If the resistance force against falling objects, is always equivalent to its weight (so they can fall faster higher up because of thinner air and lower resistance), why do meteorites burn up in the atmosphere, but rocks can be pushed way faster than their terminal velocity in water but nothing really happens?
In: Physics
Meteorites are very fast (they don’t just “fall” they come rushing). They hit the atmosphere and are immediately slowed down a lot. The heat is not friction but actually from compressing the air in front of it.
The “resistance against falling objects is equivalent to it’s weight” is only true for the equilibrium. So when the meteorite has already been slowed down, or once a falling object accelerated to terminal velocity.
The force of air friction is roughly proportional to speed squared.
In the atmosphere, the air immediately in front of the object is being compressed so fast that it’s generating enormous amounts of heat. Remember that gases change in volume with a change in pressure, while liquids do not. So in a liquid at terminal velocity isn’t causing the compression in front of it that being in a gas at terminal velocity would. It’s just pushing that volume of water away from it.
If an object starts falling while it is in an atmosphere, then the atmosphere will keep it from reaching a velocity that causes compressive heat.
But if an object starts falling from higher up, where the atmosphere is essentially nonexistent, then the atmosphere can’t slow it down at first, allowing that object to start moving much faster than it ever would if it was falling through an atmosphere. So when it hits the atmosphere, it compresses the atmosphere in front of it and that generates a tremendous amount of heat.
I don’t know how water comes into it. Rocks can’t be pushed through water at the kind of speeds they fall at. That would require tremendous amount of energy, and if you did manage to get it going it would create a lot of the same problems as falling through an atmosphere, but even more intense.
Meteors are *well* above terminal velocity in the atmosphere. They’re streaking in at 25,000+ mph, a hundred times their expected terminal velocity.
This causes intense compression of the air in front of them and the heat and shock often destroys them.
If you rammed a rock through water at several hundred times terminal velocity the violent cavitation and immense pressure would also likely shatter it.
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