Can a fly stop a train?

[Barnacle_Bill]

Probably an old chestnut, but it's been worrying me since schooldays - and that's a long time.

A train is heading down the track and a fly is travelling up it. The two collide. The fly's velocity has changed from positive to negative (or vice versa!) so at some instant in time it must have been zero. At that instant it was in contact with the train so the train's velocity must also have been zero. So for that instant the fly stopped the train. I don't think so, but where's the flaw in the logic?

The only thing I can think of is that the fly's velocity has INSTANTLY changed without going through zero, but that doesn't sound very satisfactory. Can anyone finally put me out of my misery? (I hope calculus is not involved)

[vascop]

Look at it another way: If the train stopped, it would require massive forces to stop it, and massive forces again to accelerate it back to high speed, all in the blinking of an eye. That would be magic indeed!

[lardhelmet]

This fly must be getting really tired - it's been stopping trains for two days

[Tim123]

THE TRAIN DOESN'T STOP! At the point of contact between the train and the fly, the train deforms, the deformation happening because a TINY TINY PART of the train DOES stop. That tiny tiny part then rapidly accelerates back to the velocity of the rest of the train as the deformation reverses itself, but THE TRAIN DOESN'T STOP!

[vascop]

Tim
I'm not saying the train stops - entirely the opposite in fact.

[Tim123]

Vascop,
Neither am I, but you did say that 'no part of the train stops' and that is not correct.

[vascop]

But I also don't agree with you that a small part of it stops. In my opinion NO PART of the train stops.

[Tim123]

OK, I don't think I can explain what happens any more clearly, so we'll leave it there.

[vascop]

Yes, let's agree to differ.

[Barnacle_Bill]

As the poster of the question I'm persuaded by Tim123. At whatever degree of minuteness is necessary, you end up with a fly particle and a train particle in contact with each other. If they are in contact, they must have the same velocity. At the instant the fly particle has zero velocity, so does the train particle. All the other train particles and fly particles (ie the VAST majority) can have whatever velocities they want. But, as Time123 says, because of the tiny deformation caused at the point of contact, a few train particles can be at zero velocity for a milli- billi- second.

[beso]

The only thing that really matters in changing the velocity in a collision is momnetum. In fact momentum is the ONLY property that is universally conserved from all frames of reference including relativistic velocities.

The total kinetic energy of the sysem after the collision will include the energy of the fly expressed as a temperature increase due to the deformation of the fly. However the initial momentum will be preserved.

Momentum is the mass times the velocity. Both mass and velocity are important in the interaction but in imbalance in either will render the other parameter insignificant. The 200 tonne train is not influenced much by a gram order of magnitude fly (almost) no matter how fast it is flying.

Kinetic energy is mass time the square of the velocity. Speed really counts. That is why a bullet can do so much damage yet not (as misrepresented in the movies) push the victim back.

Speed really really counts when you reach significant fractions of the speed of light. Not only does the kinetic energy increase, the mass (and hence momentum) does too! To get a perspedtive, consider these facts about the Large Hadron Collider.

[beso]

The total energy carried by the two beams reaches 724 MJ. This is equivalent to the detonation energy of 173 kilograms (380 lb) of TNT, and could be compared to the kinetic energy of a TGV (French high-speed train) running at 222 km/h (139 mph).

Loss of only one ten-millionth part (10-7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to a typical air-dropped bomb.

These immense energies are even more impressive when one considers how little matter is carrying it. Under nominal operating conditions (2808 bunches per beam, 1.15×10^11 protons per bunch), the beam pipes contain one nanogram (1.0×10-9 grams) of hydrogen, which, at standard temperature and pressure, would fill the volume of one grain of fine sand.

[McMouse]

Please could someone swat this bleeding fly

[beso]

I'm sorry but not one atom of the train needs to stop. Iron is much harder and heavier than chitin and will push its way through the fly's body in a microsecond.

[beso]

I've only just found it.

[beso]

Hey vascop they are slow to get it around here. Just want to reassure you we are not all crazy.

It is funny how stuck some become. I remember aged 16 at school and my physics teacher told the class that gasses were not compressible. I was aghast that everyone in the room but me argued on his side. I even pointed out that he used copmpressed air in his motorbike tyres but he argued that was a euphamism.

Eventually I got him to leave the room and ask other science teachers. He sheepishly returned to the room with some lame excuse about meaning fluids. He was still out of his league because fluids include gasses. I started on him but then elected to spare him further embarresment in from of the class.

[beso]

Did you know that there is one part of a train that ALWAYS moves backwards as the train moves forwards, no matter what speed the train is travelling?

[vascop]

Hi beso, That's a great story about your teacher, and thanks for the support - I was beginning to feel like a heretic!

[Barnacle_Bill]

Beso,
There's not a lot in your posts that actually addresses the question in the original post which was "...where's the flaw in the logic?" Did your physics teacher make any similar remark about answering he question?

You did say "I'm sorry but not one atom of the train needs to stop. Iron is much harder and heavier than chitin and will push its way through the fly's body in a microsecond." (I assume by "heavier" you mean "denser" - be more precise Beso, you horrible boy.) OK, consider an oak tree lying on the line. Iron is denser and harder than cellulose, lignin etc. but I think a lot of the trains atoms will have their velocities altered when the two meet. Make it a really big tree or a old sailing ship and they will stop pretty smartish. So I don't see that the properties of the the materials involved gets us anywhere.