# A fast fly hits a train and then ?

If a fly were flying very fast, perhaps .5c, and it struck a train, what would probably happen? A red-hot little fly hole through the train? A dent and a spray of fly plasma? A slow moving train? Something else?

Ignore air resistance, and consider the train to be a series of 3cm steel plates if that helps.

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According to a loose calculation (this machine does not have a decent calculator installed)
I calculate that the kinetic energy of the fly is on the order of terajoules, i.e. the collision would be like a nuclear explosion.

According to this thread, an adult housefly weighs about 12 mg. If it were traveling at 0.5c, it would have 134 gigajoules of energy, or about 34 tons of TNT. Not quite a nuclear explosion, but certainly a sizable blast. I'd think that it would just make a hole straight through the train.

russ_watters
Mentor
According to a loose calculation (this machine does not have a decent calculator installed)
I calculate that the kinetic energy of the fly is on the order of terajoules, i.e. the collision would be like a nuclear explosion.
Would it be like a nuclear explosion or would it go through the train as if the train wasn't even there? One thing about strong materials like metal and glass - they are also surprisingly brittle, they are incapable of absorbing a large amount of energy from a collision with a small object.

I think it would just puch a fly-sized hole through the train.

Hmm this is an interesting question. I think that you're all wrong.

The fly would hit the train, punch a fly-sized hole in the front wall, and disintegrate on impact.

Steel is much denser and thicker than the fly. Total amount of steel displaced by the initial collision is far greater than the mass of the fly. Application of laws of conservation of momentum & energy seems to suggest that most of the initial kinetic energy of the fly would turn into internal energy of the plasma ball (the fly + the piece of the wall displaced in the initial collision). This ball would rapidly expand as it moves past the front wall.

The end result is a front wall with a hole in it, and a chunk of the front car attached to it. The rest of the front car, the second car, etc. evaporated.

Mentallic
Homework Helper
The end result is a front wall with a hole in it, and a chunk of the front car attached to it. The rest of the front car, the second car, etc. evaporated
Fly pwns train? :rofl:

If the impact of the fly on the steel does create a plasma ball, the first plating of steel would barely slow the fly's velocity, not releasing much of that 134 gigajoules of energy as Manchot claims.

As this energy is released, in what form would it be mainly? If anyone is in the first car of the train (isolated entirely from all the other cars), would they notice anything? Would it be like a small bullet passing through a body if someone was in the way?

dst
I doubt much of the energy would be released. Most likely depends on the material properties. What kind of bonds would even hold tight at that scale?

I doubt much of the energy would be released. Most likely depends on the material properties. What kind of bonds would even hold tight at that scale?
Well, we're talking the order of GeV per nucleus, so chemicals bonds are, for all intents and purposes, not there, and even nuclear forces won't hold in the event of a direct collision. Without losing much accuracy, we can replace the fly with a bunch of 250 MeV protons.

Stopping power for 250 MeV protons in iron is on the order of 20-30 MeV/cm. So, around a third of all incoming kinetic energy would be transfered to atoms of the train on impact. In the process, incoming atoms will be deflected in all directions.