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Author Topic: Do flies stop cars when they fly into the windscreen?  (Read 1953 times)

Offline thedoc

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Paul asked the Naked Scientists:
   
A fly is flying south, a train is heading north and some folk argue that when the fly impacts on the front of the train, it has "stopped" the train because the fly's speed is reduced to zero when it impacts, and as it is in contact with the train, therefore the train speed is at at point in time zero. However, does that ever happen? Surely the slipstream would carry the fly around the train, in which case the fly flies in a circle, and is carried alongside the train before possibly alighting on the train if it were so inclined to do. Another consideration is when bird droppings splash onto a car windscreenn , but they commence from a different angle. I think i have had flies on my windscreen but then the windscreen is not vertical and I suspect the fly's body is thrust up against the windscreen so it is more of a "smudge".

Just me thinking as I sip my coffee before starting work. Sorry not a starting a cure for cancer or something more useful.

What do you think?
« Last Edit: 27/08/2015 10:29:42 by chris »


 

Offline wolfekeeper

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Re: Do flies \
« Reply #1 on: 07/08/2015 23:33:52 »
The fly kinda stops, but not really; and the train, never, ever stops.

When the fly hits the train, it more sort of ... splashes.

The bit of the fly that hits the train almost instantly starts to go the same speed as the train, but until the shockwave from that impact reaches the other side of the fly- the rest of the fly is still going the same speed it was before the impact.

There will be a point where the fly has (on average) stopped; but that doesn't mean any significant amount of the fly really has stopped; half of it is going the same speed as the train, the other half is going in the opposite direction; it has only stopped on average, it's not a solid object, and the train hasn't stopped at all.
 

Offline David Cooper

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Re: Do flies
« Reply #2 on: 08/08/2015 19:01:12 »
It's worth thinking about individual atoms in the collision. An atom of the fly hits an atom of the train and it will rebound off it, slowing the atom of the train a little. The atom of the train will then be pushed back up to speed by the atoms behind it, but it is never stopped by the fly atom, merely being slowed down. The collision between the two atoms is not instant - it takes a while as the repulsion between the two goes up and then falls again, so the fly atom reaches zero speed while the train atom is still moving towards it at high speed, this leading to them getting closer together and the repulsion forces between them shooting on up. The fly atom is then accelerated in the same direction as the train is going while the train atom is slowed further, but the train atom is not slowed to anywhere close to zero speed, and before it can lose a large component of its speed it will be hit by other atoms behind which will accelerate it back up again. There are more atoms in the fly too, of course, so they will pile in from the other side and lead to more slowing of the leading train atoms, but the most this can do is slow the leading train atoms to under half their original speed, though it won't get that far as they are in a more robust structure than the one holding the fly together. If you replace the fly with a small stone, the movement of the atoms on the train may be large enough to cause structural damage, and if there's more mass in the stone than there is locally in the front edge of the train, it could slow those train atoms to zero speed or even reverse their direction of travel. If the front was a lightweight fabric with kevlar in it, the direction of travel of those atoms could be reversed without structural damage, and then the forces holding the kevlar sheet together would cause those atoms to re-accelerate back up to the speed of the rest of the train while slowly changing the speed of the stone, so in this case, part of the train can not only stop, but go backwards for a moment (without damage resulting).
 

Offline Integza

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Re: Do flies \
« Reply #3 on: 26/08/2015 20:50:20 »
Hi ;) , I'm Integza.

The train never stops in that situation.

What happens is the following:

-The train is going North at a certain velocity (That is bigger that the one of the fly).
-The fly is going in the exact opposite direction (South) also at a certain velocity.

Because the mass of the train is so much bigger that the one of the fly and it's velocity is also superior you can imagine that it's linear momentum is enormous when compared to the one of the fly .

When the impact happens the speed of the fly changes direction from south to north almost instantaneously, this because of the enormous momentum (the tendency to keep going at the same velocity) of the train , the speed of the train is virtually the same after the impact.

I have a video in the subject of momentum that might help:


I hope a could be helpful, if the doubt remains please give me some feddback.

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Offline PmbPhy

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Re: Do flies \
« Reply #4 on: 27/08/2015 03:56:58 »
Quote from: thedoc
A fly is flying south, a train is heading north and some folk argue that when the fly impacts on the front of the train, it has "stopped" the train because the fly's speed is reduced to zero when it impacts, and as it is in contact with the train, therefore the train speed is at at point in time zero.
Whoever said that not only has no understanding of physics but no common sense either. Of course the train isn't stopped. It's momentum is so much greater than that of the train that the speed of the train is virtually the same after impact as it was before impact. The fly is at rest in its own frame of reference Sf as its moving south. Let St be the trains frame of reference. When the fly hits the train the fly experiences an enormous accelerates in the frame Sf. When the impact is over the fly is now moving in the frame
Sf whereas before it was at rest in that frame. However the train's motion hasn't changed speed (at least not by a measurable amount with ordinary instruments). So in no sense did the fly stop the train. All that happened to the fly is that it changed reference frames from Sf to St.

Quote from: thedoc
However, does that ever happen?
If you're asking whether fly's splat on the front of trains then sure, it happens. It happens on cars too. Haven't you ever had to wash off bugs that hit the front window in your car?
 

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Re: Do flies \
« Reply #4 on: 27/08/2015 03:56:58 »

 

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