So as stated above, how fast would I need to throw a water bottle (let’s say 20oz.) for it to start boiling, and why does it/ will it even boil?
In: Physics
Heat is a representation of the average speed of the random molecular motion in an object.
So lets take a box of air at room temperature. If you were to pick a molecule in the box at random that molecule will be travelling at somewhere around 770 miles per hour in a completely random direction. Some of the molecules will be travelling up, some down, some left, some right, some in every which way – all at around 770 miles per hour. The fact that they’re all travelling randomly like that causes the air itself to remain stationary since the molecules can’t move very far before bumping into another molecules, cancelling out their momentum at scales relevant to people. But if you touch that air then you start getting hit by those molecules at 770 miles per hour and they transfer their momentum to you. That transfer of molecular momentum is what we experience as heat.
So lets throw that box of air straight up at 20 miles per hour. This doesn’t increase the temperature of the air inside the box because because the motion that we added to the box isn’t random. The molecules in that air are still moving about randomly, but we’ve added non-random momentum to them
So lets take a molecule that was previously moving up at 770 miles per hour. That molecule will now be moving up at 790 miles per hour. A molecule that was previously moving down at 770 miles per hour will now be moving down at 750 miles per hour. But the increase in speed of the up molecules cancels out the decrease in speed of the down molecules, so that on average all of the molecules are still moving randomly at 770 miles per hour.
However, lets say that our box of air hit a brick wall at a billion miles per hour. The air in the box can’t continue going up – the wall stops it. But that billion miles per hour of momentum has to go somewhere, so it causes the molecules in the air to change direction. Some of those molecules will change direction to go down, some will go left, some will go right, some will go every which way. By striking the wall we’ve converted that non-random momentum into random molecular momentum. Because you now have a bunch of air molecules travelling in random directions at a billion miles per hour, that air is going to be really hot.
You might enjoy this:
[https://what-if.xkcd.com/28/](https://what-if.xkcd.com/28/)
As it says in that article, things heat up when they move really quickly through the air. It also mentions that this effect kicks in at around twice the speed of sound, which would be about 1500 mph at sea level.
But your throw would have to be much faster even than that, and I’ll let someone else handle that math.
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Speed doesn’t increase the heat of the object. Other factors like acceleration, friction on whatever that object is traveling through, deceleration, etc. would increase the heat however and given a sufficient amount of any of that it could reach very high temperatures. Think of an object entering earth’s atmosphere.
What you’re looking for is called the [stagnation temperature](https://en.wikipedia.org/wiki/Stagnation_temperature). It’s the temperature at the front of a flying object, called the “stagnation point”.
Due to some math wizardry, it all comes down to the Mach number. To boil a bottle of water you need the stagnation temperature to be 100 Celsius. To go from 20C to 100C you need your bottle to be flying at Mach 1.168, or approximately 900 miles per hour.
It’s probably likely your bottle will be destroyed before it even reaches that speed, so no, the water will never get a chance to boil.
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