# B Why doesn't the wasp die when I hit it with a fly swatter?

1. Jan 9, 2017

### Trance-

A wasp buzzes around my head and I'm annoyed. I smack it onto the floor where it is catapulted 2 meters down unto it. I watch the supposed corpse for about a minute and then I see that it is already up and about and ready to fly. How does it do that??

I imagine myself as a wasp-sized human. I jump off of over the head of my normal sized being (2 meters) and I would imagine that this would be my wasp-version's suicide. Maybe this imagination is entirely wrong but it led me to an interesting thought experiment (interesting, in my view anyway):

I have a cauldron the volume of a molten human being (let's say it's me: 2 meters of 80 kg mass). I fill it with wasps and melt them all. Once the cauldron is perfectly full of wasp-liquid, I weigh it. Either it will weigh less than the human or it will weigh more. I don't want to concern myself with the factual answer, I only want to consider both as counterfactual possibilities:
1. If it weighs more, it would mean that a wasp-sized human is lighter than a wasp so a fall of 2 meters wouldn't hurt it as much as it hurts the wasp. Is this correct?
2. If it weighs less, then the fall would probably be fatal for a human. Correct?

I want to know the physics behind it all. The weight of the wasp, the relative distance a human and a wasp must fall in order to commit seppuku, how the strength of bodies play in cushioning the fall etc. And perhaps my biggest suspicion: Pressure. What role exactly does pressure play here?

2. Jan 9, 2017

### Staff: Mentor

The density of wasps and humans is very similar - about the density of water. They won't be exactly identical but the difference does not matter.

The main point is the size: Square-cube law. If you could scale yourself down to wasp size, you would have superhuman strength (you could easily lift a multiple of your now reduced body weight), because your muscle strength decreases slower than your mass. You would also survive falls from arbitrary height (within reason: not from space), because you have enough drag so you would not get too fast.

3. Jan 9, 2017

### A.T.

And the body structure: Chitin shell with soft parts floating in a fluid of similar density, instead of hanging from bones.

4. Jan 9, 2017

### Trance-

What about pressure? Talking only about the impact, say we eject a human and a wasp at the same speed in a vacuum and they hit a surface. Who'll be hit harder?

5. Jan 9, 2017

### Staff: Mentor

That would depend on details of the biology then.

The wasp will be fine with 6.5 m/s (2 meters of free fall), a human will be fine with that speed as well.

6. Jan 9, 2017

### Trance-

Well, they'll be fine, yes, but who'll be hurt more? (I want to see things from 'pressure' point of view)

7. Jan 9, 2017

8. Jan 9, 2017

### A.T.

There is too much complexity here to tell.

To simply things, consider two balls of same material, but different size.

9. Jan 9, 2017

### A.T.

While this is important for statics (cross-section vs. weight), an impact is not quite the same: The impact force depends not only on mass (like weight) but also on the stopping distance, which scales with size (assuming the same relative deformation of a body).

10. Jan 9, 2017

### Staff: Mentor

Agreed, but it does relate to strength/weight ratio of the body structure. The OP asked about pressure. Here's an example from a creature even smaller than a wasp. Unfortunately, online references do not quote G force tolerance of the tardigrade, but they are clearly very tough. Using Newton's laws, it may be possible to estimate the peak pressure of a container of water subjected to impact.

Last edited by a moderator: May 8, 2017
11. Jan 9, 2017

### 256bits

This might be relevant or not, but it sure is interesting.
J
http://www.sciencemag.org/news/2016/02/engineers-want-know-why-it-s-so-hard-squash-roach

Last edited by a moderator: May 8, 2017
12. Jan 10, 2017

### sophiecentaur

Quantities, in general, just don't scale in the way one might think. The guinea and the feather experiment may work on the moon, where the atmosphere is virtually absent, but on Earth the electric forces between a wasp are proportional to its surface area (roughly) and the gravitational force is proportional to the Volume. The terminal velocity of a wasp is very much lower than that for a human and, even after just a couple of metres, there will be a serious difference in velocity of contact with the floor. (They say that a mouse can survive a fall off the Empire State Building.) For small invertebrates, the air is almost like a very low density liquid, through which they can swim. G forces hardly count at all, for them.
Then, once the human and the wasp have made contact, the stresses on their frames do not scale. The Bending Moment, acting on, say, a limb, will be the force times the length. The force (even in a vacuum) will be scaled by the linear dimension cubed (=weight) times the length of the lever - which gives you a fourth power factor. Then there is the thickness of the structural material, which, for the same proportions, will give another factor when working out the possible deformation or damage. So all things being equal (which, of course they are not) you can expect a factor of up to the fifth power when you try to 'scale' strengths of different sized animals and how they feel after a fall.

13. Jan 11, 2017

### sophiecentaur

I believe that the design of a fly swat, with its mesh, is based on the fact that the (very viscous for an insect) air flows through it and not round it (sometimes taking the insect with it, without contact.