I saw an ant on the table today, I brushed it aside and it fell on from the kitchen table to the floor, around 3 feet. I then saw the ant continue to move forward as if nothing happened. I feel a human may potentially break a leg if falling from a comparable height. So I’m curious, why did the ant not break any legs?
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Force is mass times acceleration. If a human and an ant hit the ground at the same velocity, they would both decelerate at comparable rates, both stopping pretty much instantly. However an ant’s mass is many orders of magnitude smaller than a human’s, so the force it feels when hitting the ground is that much smaller than the force a human feels.
The ratio of weight to surface area for an ant is much different than it is for a human. This means that when the ant falls, it only falls at about 4 mph max speed. Humans reach ~124 mph. If the table is 4 feet high, a person hits at ~11 mph. Nearly 3 times faster than the ant at max fall speed.
Next is strength to weight ratio. Human bones are incredibly strong, but we are also very heavy (compared to an ant). Ants have an exoskeleton, but are extremely light compared to the amount of exoskeleton they have. If they weighed as much as a person, while maintaining the same ratio of exoskeleton thickness compared to the size increase, their legs would probably break just trying to stand up.
So they fall slower and have a stronger support structure (related to body weight).
If you’re falling from a tall height, there are a few things that’ll decide how bad the landing hurts. If you’re falling from *very* high up, you probably have time to think about all of them, lucky you.
First is how fast you’ll be going when you land. Gravity pulls us all down with the same acceleration (more or less), but the faster you go the more the air pushes back on you, and eventually those two forces balance out – any faster and the wind pushes you back enough to slow you down, any slower and gravity speeds you back up. This is called your *terminal velocity*, and it’s the fastest you’ll ever fall under normal conditions.
An ant has a terminal velocity of about 4 miles an hour. A human has a terminal velocity of about 124 miles an hour. If you fall from high enough up, you’ll hit the ground going a lot faster than the ant will.
Why does speed matter? Because the faster you’re going, the harder it is to stop. And size matters here too – a huge thing going very fast is harder to stop (has more momentum) than a small thing going very fast. When you hit the ground it *will* stop you, and your body’s going to take the brunt of the force it takes to set the speed of something your size back to zero.
And last, some creatures really are just built different. An ant is really tiny (so the message that “hey we’ve stopped” travels through its body pretty quickly) and it’s very sturdy (it’s wearing armor!), so it can absorb the shock of the fall pretty easily. A human’s much bigger (so your feet say “hey we’ve stopped!” while your torso says “nope, still falling!”) and a lot squishier (your bones are pretty strong but they’re on the *inside!*) so it’s easier to injure yourself when stopping at the end of a big fall.
This mix of size, build, and speed means that different creatures will fall and land differently. There’s a great (and horrible) adage from J.B.S. Haldane, a biologist:
“You can drop a mouse down a thousand-yard mine shaft and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, and a horse splashes.”
As animals get smaller and weigh less, their overall durability and strength relative to their bodies tends to increase, because while the materials their bodies are made up of are the same, the weight and force applied to them is reduced. The ant weighed so little that the force on impact wasn’t enough to damage its exoskeleton.
Here’s the real ELI5 answer. an ant can never fall fast enough for it to take damage from a fall.
Ants experience less impact force when they fall, and their exoskeletons are pretty tough. Plus, their small legs can easily absorb the shock. So, they’re basically built to handle those drops.
The main things are:
– Weight, an ant is so much smaller than a human so the amount of potential force they have at any height is minuscule
– Terminal velocity, this is the top velocity something can get while falling in an atmosfere, in the case of the ant it’s relatively low density causes it to be quite slow
Here’s a great paper I recommend, it’s quite a fun and intersteing read: https://fathom.lib.uchicago.edu/2/21701757/
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