Can a fly stop a train?

Uncle Dad

A train is travelling along at let's say 100mph and smacks into a fly travelling in the opposite direction at let's say 10 mph which splatters onto the front window.

The fly's velocity has changed from -10 to 100 passing through 0 in the process. Since the fly was attached to the train's window at the time, the train must have had the same speed as the fly at that time.

In other words the fly must have, at least momentarily, stopped the whole train, right? :eek:

tacofries

wrong

Uncle Dad

great post

Victor

Uncle Dad wrote: »

In other words the fly must have, at least momentarily, stopped the whole train, right? :eek:

Wrong.

Assuming a fly can fly at 10mph in the first place and noting that:
(a) the train has minimum 100 billion times the momentum of the fly;
(b) to off-set air and rail resistances and other energy losses, the train is being propelled by the engine(s);
(c) time is continuous, not a series of points;
then in going from 10 to 0 to -100, the fly compressed / deformed (terminally!), as did the window of the train. The window of the train at the point of impact did momentarily change speed. The rest of the train did not.

Similarly, put a lead cap on a naval artillery shell and you have the basis of armour piercing rounds. The lead cap stresses the armour (without appreciably slowing the shell) and the steel shell following shatters and penetrates the armour.

The problem is the lead cap is the fly's skin.

If someone want to do the maths.

Mass of insect (typical) 0.000012 kg
Mass of train (two carriages) 100,000.000000 kg

Speed of insect 4.469444 m/s
Speed of train (two carriages) 44.694444 m/s

Momentum of insect 0.000054 kg m/s
Momentum of train (two carriages) 4,469,444.444444 kg m/s
Momentum ratio 83,333,333,333

Length of insect 0.008000 m
Interval time 0.000179 s

who_me

If it was - say - Jessica Alba's fly. And it was open. And the train driver spotted it while travelling at 100mph. Then yes, a fly could stop a train.

Otherwise, no, don't be silly.

...unless it's a big-ass fly.

jobeenfitz

So lets say the train went from 100 mph to stopping in possibly a fraction of a second. Wouldn't all the passengers be injured or dead?

endacl

jobeenfitz wrote: »

So lets say the train went from 100 mph to stopping in possibly a fraction of a second. Wouldn't all the passengers be injured or dead?

No. It's a goods train.

Uncle Dad

Victor wrote: »

Wrong.

In going from 10 to 0 to -100, the fly compressed / deformed (terminally!), as did the front of the train. Then skin of train at the point of impact did momentarily change speed.

Similarly, put a lead cap on a naval artillery shell and you have the basis of armour piercing rounds. The lead cap stresses the armour (without appreciably slowing the shell) and the steel shell following shatters and penetrates the armour.

The problem is the lead cap is the fly's skin.

Victor gets a gold star!

The solution is realising that the train isn't perfectly rigid. Only a few atoms under the impact would momentarily stop.

Good little brain exercise, eh?

GLaDOS

XKCD ran through the physics of trying to stop a train with bullets, it take s a lot!

Capt'n Midnight

I am dissapoint

Has everyone forgot that atoms don't touch each other ?
It's just electron fields repelling


Even conventional explosives will compress already very dense plutonium to twice it's density.

Victor

Of course, with laser v asteroid, the mass of just how many electrons would be needed to deflect it?

Capt'n Midnight

Victor wrote: »

Of course, with laser v asteroid, the mass of just how many electrons would be needed to deflect it?

not that many

I've heard it said that the energy needed to remove all the electrons from 1Kg of copper is about the same as splitting the earth in two,

so not that many if you do it right

Cookie_Monster

Uncle Dad wrote: »

Victor gets a gold star!

The solution is realising that the train isn't perfectly rigid. Only a few atoms under the impact would momentarily stop.

Good little brain exercise, eh?

nice backtrack

Uncle Dad

Nice try.

OCorcrainn

The whole meaning of the phrase "an unstoppable force meets an immovable object" is lost on you I gather.

Pegmatite

Uncle Dad wrote: »

Victor gets a gold star!

The solution is realising that the train isn't perfectly rigid. Only a few atoms under the impact would momentarily stop.

Good little brain exercise, eh?

No!!