A fly hitting a train

We prob all know that if a fly flys into a train, a part of the train must stop for a split second because the fly's motion must be reversed.

But how can a fly effect only a part of a sturcture? Surely the fly vs speeding solid metal does not seem a fair fight? Can it really have an impact that it would stop a small section of the train? I know that it must be the case but it is very hard to see, can someone explain?

 

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No, no "part" of the train is "stopped" even for a micro-millisecond. The forward motion of the fly (it's inertia) is instantly converted into heat energy upon impact with the train. Since the train's inertia is larger (by a factor of millions) the train, of course, is completely unaffected.

 

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Except for that little red splotch it now sports, of course.

 

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Ha!

 

The train is slowed down by an immeasurable, but calculable amount.

 

Agreed. But it would be SO small that I don't think I'd even want to calculate it.

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Don't think about speed, but momentum. If you think that the fly is travelling one direction, at speed X, then hits the train, and is if then going the opposite way at speed Y, then yes, it implies at some point that the flies velocity must equal zero, and as it is in contact with the train, the train must also stop.

BUT, if you consider momentum, all you do is sum the momentum of the two objects, so;

MassTrain x VelocityTrain - MassFly x VelocityFly = post impact momentum.

Then divide the new momentum by the mass of the train, and you see how much the train actually slowed down.

Impulses are kinda difficult to get your head around but if you think about momentum, it's easier.

 

Quite right - turns out to be just about a billion.

 

Although if it was a very large fly and a very small train..........

 

If it was a model train slamming into a carpenter bee, you might be able to measure it.

 

In France they have these really big flies. Looks like bluebottles half the size of your fist. I think they click, or buzz. Odd things. What exactly are they?

 

Nah, the fly goes "splat" I've seen it a million times with all kinds of vehicles. Fly goes "splat" ev'ry time.

 

You aren't looking at it from the point of view of the atoms of the train at ground zero. They go splat too.

 

If you are referring to fly blood, it is milkish in colour, or sometimes blueish, but I haven't seen it red yet.

Unless it was a female moquito that just ate hits the train.

 

how about a really fast train? could a big insect put a dent if it hit square on the pointy tip of a TGV going 300mph? if not an insect, what would it take, kicked up gravel?

 

Every time I have ever swatted a fly with my fly-swatter, there is a big red blob left where I swatted it.

 

All I see is red mosquitos splattered on the windshield...why red?...well thats my blood of course...thats what they get for stealing part of me.

 

That's awfully small... shouldn't it be 10^-15 meters?

 

And this is why objects never touch, they merely levitate against each other at a distance of 1 angstrom, or so. A Short History of Nearly Everthing FTW!