[Seany (OP)]

You've seen rocks floating in the air.  Very small rocks, with a lot of surface area in proportion to their mass.  They are called dust.

Dust only appears to be floating from our perspective - the dust is in fact falling through the air (albeit, as you say, very slowly), but small pockets of air air rising, and the dust is sometimes falling slower than the pocket of air it is in rises, so the nett effect from our point of view is to see the dust itself rise.

Even humans can do this - in a tornado - it is just that to a small spec of dust, even the ordinary turbulence of warm air will feel like lots of tornadoes blowing about.

Ok, now you make my life hard. []

[Seany (OP)]

Soo... Assuming that the terminal velocity for a human is 120mph, and for an ant is 0.25mph. 0.25mph is roughly the speed of.. Ok, nothing that slow.. Yes, I can see how it won't affect the ant at all, seeing that our hands swatting a fly would probably be like atleast 1mph and it sometimes doesnt even die.

But, 0.25mph for an ant. How fast would that feel for an ant? Because, thinking as a human, 120mph is FAST. Even the wind at 120mph means shed roofs, fences falling down. So would 0.25mph still be QUITE fast for an ant? But won't affect them hardly as much as 120mph?

[another_someone]

Soo... Assuming that the terminal velocity for a human is 120mph, and for an ant is 0.25mph. 0.25mph is roughly the speed of.. Ok, nothing that slow.. Yes, I can see how it won't affect the ant at all, seeing that our hands swatting a fly would probably be like atleast 1mph and it sometimes doesnt even die.

But, 0.25mph for an ant. How fast would that feel for an ant? Because, thinking as a human, 120mph is FAST. Even the wind at 120mph means shed roofs, fences falling down. So would 0.25mph still be QUITE fast for an ant? But won't affect them hardly as much as 120mph?

I would say that your hand swatting a fly will be considerably faster than 1mph.  4mph is about normal walking speed, and you can easily slap someone on the back while they walk past you, so you hand must be travelling faster than 4mph.  Running speed is between 11mph and 22mph, depending on how fit you are, and whether you are running a marathon or a sprint - I imagine one can slap someone on the back as they run past you, but you may have to move a bit faster to do it.  Ofcourse, the longer the reach of your arm, the faster your hand can travel, and if you extend your reach by using a fly swap (or even a rolled up magazine), then the speed will be faster yet.  When you crack a whip, the crack that you hear is when the tip of the whip actually exceeds the speed of sound.

http://www.sciam.com/article.cfm?articleID=00088B33-8E92-1CEE-93F6809EC5880000

It takes a dexterous hand to coax a whip to crack. Now researchers report that they have discovered the mechanism responsible for the startling sound. It has long been thought that the crack results from the tip of the whip traveling fast enough to break the sound barrier and create a sonic boom. But the new findings suggest otherwise. Apparently, it's the loop in a whip that is the real noisemaker.

Though by no means a master whip cracker, Alain Goriely of the University of Arizona was nonetheless intrigued by the phenomenon and set out to study it at a theoretical level. Together with Tyler McMillen, a graduate student in applied mathematics, he modeled the behavior of the leather strips in a paper to be published in Physical Review Letters. Previous whip work (one of just three papers on the subject in the past century) had resulted in the puzzling observation that the sonic boom occurs when the tip of the whip is traveling at about twice the speed of sound. But if the tip were truly the cause of the crack, why wasn't the sound heard earlier, when the tip first reached the speed of sound? Goriely and McMillen's calculations have revealed the answer. "The crack of a whip comes from a loop traveling along the whip, gaining speed until it reaches the speed of sound and creates a sonic boom," Goriely says. He notes that even though some parts of the whip travel at greater speeds, "it is the loop itself that generates the sonic boom."

Although the whip's tip has lost the distinction of being the source of the menacing crack, it is still a force to be reckoned with: according to Goriely's calculations, "the tip can reach speeds more than 30 times the initial speed [of the whip]."

Winds of 120mph can shed roofs because our roofs were not built to withstand winds of 120mph, because they are not regarded as normal events, and when the abnormal happens, we fail to cope.  The issue with an ant must be regarded not with regard to the size of an ant, but with regard to what is normal for an ant.  If it is normal for an ant to encounter speeds in excess of ¹/₄mph, then it will not regard it as exceptionally fast, because it is within its notion of normality.

[Seany (OP)]

Thanks for that. So the whip travels at like 30,000mps?

And also, don't you think an ant dropped from a building, would have a speed of more than 0.25mph? I was thinking more like atleast 10, even though it is a small fella.

[another_someone]

Thanks for that. So the whip travels at like 30,000mps?

No, more like 660mps (about 1480mph).

And also, don't you think an ant dropped from a building, would have a speed of more than 0.25mph? I was thinking more like atleast 10, even though it is a small fella.

I did say that I was taking the ¹/₄mph merely as an extrapolation of your pro-rata argument, and I had no confidence in its realism.

I do agree, that I would, from empirical observation, guess it to be higher than that (but still well below that which a human would reach).

The major difference that matter is that I would expect that an ant dropped from table top height would already have reached terminal velocity by the time it reached the ground, which a human would drop the height of over 500m before reaching terminal velocity.  A drop of 500m is very rare for ordinary humans, so they have not been designed to survive it, a drop from table top height would be far less rare for an ant, and so it is reasonable to assume it has been designed to withstand that drop.

Incidentally, looking up references, I see that the terminal velocity for a human body, as stated as 120mph, must be qualified - this is the speed at which a freefall parachutist will reach, with arms and legs outstretched to maximise drag.  Diving head first, with arms by your side, you can reach speed of up to 200mph.

Hairy animals, like cats, have an additional advantage in that the hairs on their body, if they stick out, will significantly add to the drag factor, and so further reduce their terminal velocity, which is why cats can withstand substantial falls.

[Seany (OP)]

But how can you assume that a drop from a table top for an ant is not rare for an ant?

After all, that is like 1000x taller than their height. And ants live in a 2-Dimensional world unlike humans in a 3-Dimensional world. Surely ants aren't used to being "dropped" ?

[another_someone]

Actually, ants don't live at all in a 2 dimentional world - they crawl over all sorts of things, and generally are good climbers - but what goes up, must come down, and sometimes precipitouslly.

There are almost 12,000 species of ant, so one has to ofcourse be careful about making any statement that is true of them all, but certainly, I have often seen ants crawling along table tops, and sometimes will fall off the edge.

http://en.wikipedia.org/wiki/Ant

[Seany (OP)]

MM lol thanks. Are you sure they didn't die falling off the table edge? []

[Bored chemist]

OK, so it's not just physicists who calculate absurd assumptions like the free fall speed of spherical ants; Bored Chemists do it too.
This page
http://en.wikipedia.org/wiki/Stokes'_law
tells you what the formula is for the terminal velocity (under a set of assumptions that may not be true here).
The terminal speed increases with the square of the radius.
Since I could roll up into a ball about half a metre across and an ant could bundle itself into half a centimetre we are talking about a radius ratio of about 100 to 1. That makes the terminal velocity for the ant about 1/10000 of mine.
120MPH is about 53m/sec (this makes no real difference but converting it into metric makes it look more scientific)  [].
So our ant should fall at half a centimetre per second. OK it looks like that's a bit low to me, but I did say I was making some assumptions.

There's another big matter to look at here. Ants and people are held together with exoskeletons, ligaments and stuff. All those are, in turn held together by chemical bonds of some sort. Breaking me or the ant needs energy in order to disrupt those chemical bonds.
At 53m/s every kilo of me carries about 1400 Joules of kinetic energy.
Each Kg (OK, I know ants don't weigh 1 kilo) of the ant at 0.053 m/sec carries just 0.0014 Joules.
 When we hit the ground the ant has a lot less energy to dissipate than I do. That's why it survives. The splat per unit weight varies as the fourth power of the radius. (It's not often you see 4th order terms in real life)
I think a typical cat is about 5 kilos in weight; a typical human is about 70 so a cat weighs roughly 0.07 times as much. If we assume the same shape and density (which is absurd, but it's as good an approximation as I can think of) it must be about 0.4 times as big as a person. (That would mean that compared to a spreadeagled human who cover about 2 metres diameter a cat would be about 0.8 metres front paw to opposite back paw which seems a bit big) If that's true then, at terminal velocity, it possesses about 3% of the stored energy of a falling human. If the cat's smaller than that it does even better. I think that may have more to do with cats' survival than their fur (though the fur must help).
Similarly you can look at the few people who have survived falling from planes without parachutes; they generally landed in trees or snow (or both). In this way the energy was dissipated by breaking twigs and squashing snow rather than by the body.

[another_someone]

So our ant should fall at half a centimetre per second. OK it looks like that's a bit low to me, but I did say I was making some assumptions.

¹/₂ cm/sec. (not even SI, but CGS units []) amounts to about 30cm per minute, or about a foot per minute.  From 30,000 feet, that is 30,000 minutes to reach the surface, or just under 8 hours.

Not that I actually thing 1 foot per minute (about ¹/_100thmph) is a credible speed - so from a table top that is 3 foot off the ground, it should take 3 minutes to reach ground to reach the ground (this is not even allowing for the time to accelerate to terminal velocity).  Last time I saw an ant drop from that height, I did not recall having to wait that long for it to reach the ground.

[Seany (OP)]

Lol anyway Bored Chemist, thanks alot for that, now I understand why it wouldn't die. Because of the energy which the ant has is so much lower than that of a human.

So is there no way that an ant can die, from falling?