The Fly and the Train

[Whickerman]

This is straight from the pub, so no apologies for this sounding daft.
Is this true...---->?
If a fly hits a train head on, it obviously loses. BUT - the force it's exerted on the train must surely have caused the train to slow down, even though we could probably never measure it as it's such a small relative force.

[Clanad]

Sure... Newton said and proved that a body in motion tends to remain in motion (in a straight line) unless acted upon by an outside force... and secondly (pun only slightly intended) Newton gives us the fomula F = ma which, when applied to your scenario and given sufficient information could actually solve the ever so slight affect said fly has on said locomotive...You, of course, know the last thing to go through the fly's mind a nano-second prior to his demise, don;t you?

[Whickerman]

lol clanad!

Thanks!

[Buenchico]

Oh, how I wish I was back in my old job (running a railway station). I used to love finding new ways of giving the reasons for delays. ('Leaves on the line', for example, would become "track adhesion difficulties due to vegetation debris on the railhead").

I'd really enjoy announcing "We apologise for the delay to this service. This was due to the locomotive suffering multiple collisions with calliphorae vomitoroa"
- or, in layman's terms, "It collided with a few bluebottles" ;-)

Chris

[MatydaLover]

You can make an estimate of the effect. The total momentum of the train and the fly before and after the collision will be the same.

Let's say the train is 500 tonnes at 100 metres per second, and the fly is 1 gramme at 10 metres per second.

The total momentum is 49999999.99 kg ms(-1).

After the collision, the mass of train now includes the fly so its new v is 49999999.99/500000.001 ... or about 99.9999998 ms(-1).

So it is slowed by about one part in 50 million. It would be about a second late on a journey lasting a year and a half.

(i'm happy for someone to come up with a correction or two, it's easy to lose count of the decimal places!)

[heathfield]

The original argument (as I recall it from my youth) is that, since the fly must come to a stop before it reverses direction, then the train, too, must come to a stop.

Obviously the train doesn't stop, but nor does it slow down.

Only a molecule of two of the steel on the front of the engine would be affected, causing a miniscule denting, probably too small ever to be measured. If anything stops, it's only these couple of molecules. The compression of the steel would create a tiny amount of heat that would dissipate immediately, and, given the elasticity of the steel, the dent would bounce back to to form near enough the shape of the original smooth surface.

That's how I worked it out, all those years ago.

[sddsddean]

Just as an aside, in my 'Would you believe it?' book it is stated that if a fly lands on the handrail of the QE2, the ship lists by the distance of the width of one atom! Would it really make that much difference?

[wildwood]

Does using 1 gram hitting 100 tons to calculate the reduction of the train's speed take into account that the fly's surface is considerable softer that the train's?

[heathfield]

It's the mass of the object hitting the train that counts, Wildwood, not what it's made of. As Clanad points out, Newton's 2nd Law states that Force = Mass x Acceleration. Note that density isn't a consideration.

[Theland]

So, what's he best way to swat a fly?

[shylock73]

With a Flyswatter Theland as the fly doesn`t feel the air pressure caused by the swatter in time due to the fact that the swatter is full of tiny holes allowing the air to pass through, but you gotta be blooming quick even so. I Bet you wish you hadn`t asked now.

[nightmare]

if you do not have a fly swatter to hand when you actually need one, it is much more effective to attempt to swat the fly simultaneously from two opposite directions.
This appears to outwit the fly's reactions.
Using two large, heavy books gives a very satisfying thud, as well, as the fly gets laterally compressed to a lethal degree - or splatted, in common parlance.

[weggiewabbit]

I�d recommend flicking a towel at the fly as it goes past � this can be very effective!

[Muppit]

I hate to be pedantic, but I don't like to see people making flase (or inaccurate) statements....

It does matter what it's made of, the fly is like a crumple zone on a car... It would compress, increasing the deceleration time, reducing the overall force. As we know, the change in momentum (MV-MU) is equal to Force x Time (Ft); Ft=MV-MU, F=(MV-MU)/t, to as t (the time taken for the complete deceleration of the fly through it's inevitable splattering), increases, the force exerted on the train decreases proportionally. So, if the fly has just eaten his dinner, he'll be stopping the train for a bit longer than if he is on his way out for dinner.

[lola57]

to come back to how to kill a fly.I found out, that a vacuumcleaner is very helpful,when you have no flyswatter handy.Suck em up!lol
The train slows only down,when it hits a huge flyswarm.Otherwise it doesn't matter.

[Sildromas]

Yes it's true. The forces (action and reaction pair) exerted in enough to send the fly to kingdom come, but no enough to slow down the train significantly.