And if you plot the velocity vectors the fly comes to zero speed relative to the ground - while in contact with the train. So, logically, if they are in contact and the fly has zero velocity so should the train - hence the "conundrum".
Again, the pieces of the fly are theoretically only actually at zero velocity for an infinitely short duration – a single instance of time. If you plot the velocity of the fly (or a single atom of the fly) vs. time on a graph, the fly’s speed will stay constant, then begin to slow down as it begins to hit the train, then pass through zero and continue to go negative as the fly begins to accelerate in the opposite direction. But the plot will only pass through zero at a single value, not a range of values. That single value is the one, infinitely small duration of time at which the fly’s speed is actually zero. It’s not a billionth of a billionth of a second, because that would imply that the fly’s velocity passed through zero over some range of values (ie, x to x+10^-18), rather than at a single value. It might seem nitpicky, but it’s the key to understanding why there’s no problem here, even with hypothetical perfectly rigid flies and trains. edit: No. The fly's velocity is only zero for an infinitely small duration of time. Since the train would not be expected to move at all over an infinitely short duration (multiply some velocity in meters/sec by zero seconds and you get a distance of zero meters traveled), there is no conundrum. There is an infinitely short duration in which the fly has a velocity of zero and the train has some non-zero velocity. Neither the fly nor the train will move in that instant, because nothing can move any distance during a single instance of time, regardless of velocity.
If it's at zero AT any point it's at zero - and that point happens to be when it's in contact with the train - therefore the train should also be at zero (for however short a time - which is not the case since the train does not suffer massive deceleration).
Yup, but when most people are asked they tend to think of "fly = 1 unit" "train = 1 unit" and get confused... Please Register or Log in to view the hidden image!
I’m trying to not be rude, but it’s reaching the point here where I’m tempted to just say “go take a calculus class” and give up. No. It is possible for the fly to be at zero velocity and the train to have some non-zero velocity at the same infinitely short instant of time, even thought they are in contact with each other. It would not be possible for that to occur over any non-zero duration of time, but we’re talking about a length of time that is exactly zero seconds long. Your intuition is fooling you here.
You are quite correct. Thanks. Nothing wrong with being nitpicky. The point I was trying to make is there is also no problem because the fly is loaded with acceleration when it reaches that smallest (planck?) moment of zero velocity.
I think you and Oli are pretty much agreeing here though. The fly and a microscopic part of the train are at zero velocity for an infinitely short duration of time (the moment of inpact). I dont see how you two are in disagreement about this.
If you're going to allow the train to deform, you don't have to have an infinitely small duration of time in which the fly is stopped but the train is moving. You can have some non-zero duration during which the fly (and the front of the train) is completely stopped, but the rest of the train is still moving. In any case, I think we can all agree that the train/fly system that Billy proposed don't pose any serious metaphysical problems for physics or philosophy.
Ofcourse the deformity would probably be non-existent here but we have to make the reasoning work for heavier objects as well. A little bit of both happens in reality i guess. I think that can be savely said Please Register or Log in to view the hidden image!
No it hasn't. If you look in detail you'll see the surface of the fly decelerate, stop, and accelerate as it approaches the train (this is caused by repulsive forces between the fly's and train's molecules that build up when the two surfaces come close enough to one another - they're never actually "in contact"). The same thing happens throughout the fly until its entire corpse (by now thoroughly squished by the compression wave) has matched the velocity of the train.
So a fly couldn't stop a train but a table tennis ball could? Please Register or Log in to view the hidden image!
i dont understand, i have read it about 8 times and im still stumped. i mean i think i get what is said kinda, but i dont understand how the train was stopped by the train. peace.
Another very simple point is conservation of momentum. The net momentum of our fly/train system MUST always stay the same, therefore, the train and the fly cannot ever both be stopped. -Andrew
the question is what speed does the fly has to travel to stop the train from moving...my guess is faster than light
how the fuck does a fly stop the train though?, i really dont understand this atall. the fly cant stop the train, they can both just stop or the train cans plat the fly, but the fly cant stop the train. thats stupid. ok i will test this theory and if the fly dies then you guys owe me a fly. peace.
yeah lala land, its simular to the place where flys are made out of anti-matter. woo-woo land. peace.
