Why Don't Insects Get Hurt When they Fall??!!

“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, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes." -J.B.S. Haldan 

Image from TeePublic

Have you ever wondered why insects don't get hurt when they fall?! I know I've flicked bugs off of my shoulder and seen that they're totally fine when they land. If you flick a bug off of your shoulder, relative to their length, they're falling from the same height as a human flicked from the top of the Statue of Liberty! No human would survive that! How the heck do bugs??!!

Before you read further, check out this awesome YouTube video which demonstrates and explains the topic really well!

The answer is really quite wonderful to wrap your mind around, and is related to many other seemingly unrelated phenomena like why potatoes cook faster when you cut them into smaller pieces and why elephants have such large ears! (Which I won't explain here, but you might be able to figure out! Hint: It has to do with cooling themselves...)

I'm going to try something unusual with my explanation here and go backwards. Let's see how it works!

Turns out bugs don't get hurt when they fall primarily because they don't fall as fast as a human (and also because, even if a bug and human hit at the same speed, a bug's smaller mass means that its momentum and therefore force experienced would be less). But why don't bugs fall as fast as humans?

When you fall there are two important forces that determine how fast you move, the downward force of gravity and the upward force of air resistance. If the downward force of gravity is a lot larger than the upward force of air resistance, you speed up quickly and wind up falling at a faster speed. If it's only a little larger, you speed up slowly and wind up falling at a slower speed.

Turns out that the downward force of gravity is a LOT larger than the upward force of air resistance for something as big as a human. And for something as small as a bug, it's only a little larger. How come?

Turns out the force of gravity depends on how much volume (space) your body takes up (it actually depends on mass, but since more massive living things tend to take up more space, we can say it depends on volume). Humans take up a lot more space than bugs, so humans experience a much larger force due to gravity. It also turns out that air resistance depends on how much surface area (effectively how much skin) an object has. Humans certainly have much more "skin" than bugs...

But, as I mentioned, it turns out that a falling object's speed depends on BOTH the downward force of gravity and the upward force of air resistance. So it depends on both the volume AND the surface area. If you have a lot more surface area than volume, you'll gain speed slowly (think of a falling piece of paper). If you have a lot more volume than surface area (crumple that paper up and drop it again!), you'll gain speed quickly!

Turns out, as objects get bigger, they get a lot more volume than they do surface area! (For a sphere, volume grows as the cube of its radius while surface area only grows with the square of the radius).

So a bug's volume is relatively small for its surface area, and a human's volume is relatively big for its surface area.

So the force of gravity on a bug is relatively small compared to the force of air resistance, and the force of gravity on a human is relatively BIG compared to the force of air resistance.

So a bug falls slower and a human falls faster.

So a bug is fine and a human goes splat! :)

More on the topic:
NyTimes on Falling Bugs
J.B.S. Haldane Essay: "On Being the Right Size"


This post was initially inspired by Paul Hewitt's "Conceptual Physics" which does a wonderful job discussing the effects of scaling and the surface area to volume ratio.