Can a fly stop a train?

Question

A fly moving due North at 40 mph collides with a train moving due South at 80 mph.

An instant later the fly is travelling opposite to its original direction.

The fly must have been decelearted to zero velocity before it could be accelerated in the opposite direction.

If the fly is at zero velocity the train must also, at that instant, be at zero velocity?

Answer 1

Nope...force is what you are looking to determine. Force equals mass x acceleration. The fly would have to travel MUCH faster to have any impact on the train. Also, the fly, once it made initial contact with the train, doesn't decelerate all at once...parts of it continue moving after other parts stop, and some parts start moving in East and West directions...SPLAT!

Answer 2

It's nonsense. There are lots of maths and equations that will help you disprove what your teacher says, but the main reason why this is bunkum is because in order to stop the train the fly must have the same energy as the train. As energy is a function of a body's mass and velocity you can see that the train will have a massive amount of energy compared to the fly which will have a tiny amount. All that will happen at the point of collision is that the train's energy will be depleted by the amount of energy that the fly has. Plus, if the train is actually stopped by the fly then how does it then continue with almost undiminished speed? Where does the energy come from to accelerate it back to 120mph? This is the kind of received wisdom that seems to get taught in schools in place of real science. However, there is a grain of truth in it and it's all tied up with the Theory of Relativity. If you were to observe the collision at a massively slowed speed you would not see the train stop. However, there probably is a point where the fly's speed is zero, but only for the length of time it takes the train to travel slightly less than the length of the fly's body as the fly is squashed flat in the collision. However, just because the fly has momentarily stopped moving it doesn't mean that the train has. But well done for questioning what your teacher has told you.

Answer 3

The answer is yes - the fly can stop the train (but only under one condition)

The fly is travelling at half the speed of the train in the opposite direction. Both the fly and the train have momentum (mass x velocity). Assuming the fly sticks completly to the train on impact, it would be possible that the train will be stopped - if the momentum of both objects is the same. Therefore, the fly would have to have a mass of 2x that of the train. this would be a very large fly - no doubt a mutant fly. No such flys are currently known to exist, so given current constraints on the average mass of a fly the speed of the train would not measurably change as a result of the colision

Answer 4

You have started skimming the edges of Heisenberg's uncertainty principle which basically says that you can't know the position and velocity of anything at the same time.

But to answer your question, when the fly hits the train there is indeed a reaction to this as it's mass is changed in direction there would be an indetectable deflection in the metal on the front of the train. In effect this metal would be moving in the same direction as the fly for the briefest of moments. The train as a whole would continue going the same direction. This deflection in metal would of course spring back and the shockwave would propagate through the train until friction robbed the shock of it's energy, just like throwing a stone in a still pond.

Of course the other forces acting on the train would far overpower the fly's inertia.

Answer 5

Technically yes. If the momentum of the fly and train are equivalent, a collision between the two will result in both fly and train decelerating to a standstill. Momentum is mass times velocity but as the train has significantly more mass than the fly, the fly would have to be doing incredible speeds and at such speeds is likely to disintegrate.

Answer 6

You answered your own question.

The fly does not decelerate to zero mph. the instant it hits the train it velocity changes from 40 mph in a northerly direction to 80mph in a southerly direction. This instantaneous change in velocity results in a small bloody mess on the front of the train.

Answer 7

How about a simple answer

No, because of the law ofconservation of momentum

If the fly stops, and the train does too, almost all momentum was lost to nowhere.

Momentum=mass times velocity

Answer 8

The fly receives force from the train on impact, that's why it stops and reverses.

The fly could stop the train if it weighed twice as much as the train, as it's flying at half the speed. Due to the crash though, you wouldn't be able to distinguish the fly from the train....

Answer 9

If the fly landed on an exposed wire on the train, it could short circuit the electronics and bring the train to a standstill. Or the fly could lodge in the drivers eye causing him/her to swear a lot and they could stop the train to seek medical help. It happened me once, I was riding my motorbike without a helmet one fine summers day and a fly give me the biggest shiner ever! Lots of swearing!!!

Answer 10

Can a fly stop a train?
Maybe If the fly is flying faster than the blazes of hell, and I mean fast, at the speed of light or faster.
Then the E becomes Mass (C).