Does the fly stop the train.

[pullup]

Actually, their relative velocities are 0 from the time the fly moves along with the train until the time someone removes said fly's carcass from the train. That will most likely be much longe than a nanosecond.

eh, true, true.....was thinking that it would bounce but on second thought it wouldn't!!!!

 

[ppragman]

P=mv

(m1v1)i + (m2v2)i = (mtvt)

Where 1 = train, 2 = fly, t = the two together.

The fault in the logic is looking at the fly independently from the train. However, since the fly joins the train, you must look at the system as a whole.

Let's say, for argument's sake that the train weighs 1000 kg and the fly weighs .1kg. Let's call the train flying at 30 m/s and the fly is flying at .5 m/s

1000kg * 30 m/s = 30,000 kgm/s
.1kg * .5m/s = .05 kgm/s

So the system must have 30,000.05 kgm/s of momentum.

30,000.05= 1000.1kg * v

v = 29.99705 m/s.

So the fly slows the train down an almost unnoticeable amount.


There's no stopping the train for any sort of nanosecond bs.

:yeahthat:

I'm glad you did that cuz I don't really feel like explaining all that.

 

[Nismaxdan]

Daag, some people on here are either really smart or in need of a really good lay.

 

[Boieng747pilot]

AAAAAHHHHHHHHHHHHHH Trying to prep for an exam on Monday.... should NOt have read this thread. :banghead:

:rawk:

 

[gevo]

If you were to put your finger on the inside of the windshield where you hit the fly, you'd probaly feel something when you hit it, which means the fly may have stopped a few molecules of the windshield, but that doesnt mean it stopped the whole train.

hahaha.. wow, i haven't heard this one yet.. umm.. i'm not sure to be honest.. glass IS technically a liquid still.. it could elasticaly deform...

 

[gevo]

The key here is to look at the speed as a relative variable. From the viewpoint between the fly and the train then in reference to each other at that exact nanosecond then they are traveling 0mph. However, from an outside reference the train would obviously not hit 0 mph.

Yeah, sorry man I couldn't really undersatnd what you said.. kind of circular reasoning.. But you both admitted to the train stopping and not stopping

 

[gevo]

P=mv

(m1v1)i + (m2v2)i = (mtvt)

Where 1 = train, 2 = fly, t = the two together.

The fault in the logic is looking at the fly independently from the train. However, since the fly joins the train, you must look at the system as a whole.

Let's say, for argument's sake that the train weighs 1000 kg and the fly weighs .1kg. Let's call the train flying at 30 m/s and the fly is flying at .5 m/s

1000kg * 30 m/s = 30,000 kgm/s
.1kg * .5m/s = .05 kgm/s

So the system must have 30,000.05 kgm/s of momentum.

30,000.05= 1000.1kg * v

v = 29.99705 m/s.

So the fly slows the train down an almost unnoticeable amount.


There's no stopping the train for any sort of nanosecond bs.

Hmm, I agree with the work you did.. But your analyzed the wrong portions of the problem. You analyzed them seperately, and then after hey have come together and after the fly has 'changed' its direction. You would have to analyze somehow theenergy of the fly and the train at the moment of impact, not any amount of time after wards as you suggested above.

The meat is to argue the logic of how the fly and the train come together. I would think.

 

[gevo]

AAAAAHHHHHHHHHHHHHH Trying to prep for an exam on Monday.... should NOt have read this thread. :banghead:

:rawk:

hahaa.. Also studying for an exam on Monday.. shouldn't have made this thread... .yet...

 

[gevo]

Wow, after four excedrin hopefully I can think straight. I would propose to you that the fly is never attached to the train in the way you present your question. The dynamics in the change of direction are borne by the fly and not the train. The two never become one.

Calcapt, I believe you are the only one who is really close... This is what I believe should also be questioned, the fasion in which the two objects join.

The best explanation I have come up with still has a loophole. That is, the fly has restitution, meaning it is not a solid object... for example a billiards ball has a very low coefficient of restitution, not all that much 'give' to it.. and a fly is much softer obviously. SO, the time it takes the fly, or all of its pieces once it hits the train, to decellerate, in this time is where the energy is transferred... during the time it takes when the fly first touches, then proceeds to splatter, until its every piece has changed direction..
BUT! I still can say that since every molecule on the fly eventually DOES change direction, and these molecules are attached to the train (if it weren't it wouldnt have a reason to change direction would it?) still we are back to the original question, why shouldnt the train ahve stopped as well.. two bodies which are moving aas one must move at the same velocities right?

by the way, my dynamics professor introduced this to the class, then laughed at us and never spoke about it again!

 

[darrenf]

C

by the way, my dynamics professor introduced this to the class, then laughed at us and never spoke about it again!

I can understand that.

 

[Chris_Ford]

two bodies which are moving aas one must move at the same velocities right?

Nope. There's your logical leap.

Newton's laws all deal with Force.

F=ma

there is no velocity component. With the change of direction, the fly is accelerating, and Newton's laws remain intact.

Seriously, this is dumber than the treadmill thing.

 

[casey]

Hmm, I agree with the work you did.. But your analyzed the wrong portions of the problem. You analyzed them seperately, and then after hey have come together and after the fly has 'changed' its direction. You would have to analyze somehow theenergy of the fly and the train at the moment of impact, not any amount of time after wards as you suggested above.

The meat is to argue the logic of how the fly and the train come together. I would think.

take some more physics classes.


if you want to look at it from an energy standpoint you need to look at the impulse. We know some things, for example the train starts at 30 m/s, and ends at 29.99705 m/s and has a mass of 1000kg. The fly starts at -.5 m/s and ends up at 29.99705 m/s and has a mass of .1kg.

impulse = mass (vf-vi). which for the train and the fly gives us -2.95 and 2.95, which is expected per newton's 3rd law (forces being equal and opposite). So now you have the impulse of the collision, 2.95 Ns, and if you want to find the force imparted you need to know how long the collision takes. since we made up all the other numbers lets say the collision takes place in 1 ms, or .001 s. J=F*t, 2.95=F*.001, F=2950N.

Taking that further, F=ma so we can derive acceleration of the fly and train.

for the train 2950N = 1000kg*a, a =2.95 m/s^2.
for the fly, 2950N = .1kg*a, a = 29500 m/s^2.
g = 9.81 m/s^2.

the train experienced .3 g's of deceleration from the impact with the fly, while the fly experienced 3000 g's of acceleration which is major contributing factor in its death.

disclaimer:
Its been a long, long time since ive taken a physics class.

 

[darrenf]

for the train 2950N = 1000kg*a, a =2.95 m/s^2.
for the fly, 2950N = .1kg*a, a = 29500 m/s^2.
g = 9.81 m/s^2.

I like the equation, but that is an absurdly light train, and an equally as unlikely heavy fly.

 

[casey]

I like the equation, but that is an absurdly light train, and an equally as unlikely heavy fly.

im just using the fictional numbers chris_ford used in his momentum equations, that way all the numbers in the thread work together.

yes, the train would be much, much more massive, and the fly smaller but the numbers demonstrate (hopefully) the concept gevo is looking for.

 

[Velocipede]

My brain hurts!

 

[SteveC]

Does graphical work better than equations for some of you?

Notice the fly's velocity passes through zero while the train's does not.

 

[averyrm]

This might help - The fly and train arn't acting as one unit until the fly joins the velocity of the train, by splating spectacularly. As it splats it expends most of its energy in the z axis (looking at a side view) and some of its energy slows the train down. You're getting too wrapped up in thinking they're the same thing.

If you look at SteveC's drawing (seriously, you took the time to draw that?? ) you can't consider them one unit until the fly's velocity catches up with the train's - it's moving by deforming the train and itself.

 

[gevo]

I can understand that.

hahahah... yeah..

umm guys, Casey... I am not actually proving that the train stops.. thats absurd! I am merely playing a kind of 'devils advocate' role to see if anyone can actually be convinced.. it just gets people thinking.. I've taken plenty of physics courses thank you..
The point was for the leap of logic to be pointed out, and Chris pretty much got really close.. BUT!! AGAIN!!! this isn't an 'equations' problem, its a logic problem.. this is all

Its still weird to think about it, in my opinion.. But, uhh.. thanks for all the hard work with the diagrams and everything.. hahaha:crazy:

 

[gevo]

This might help - The fly and train arn't acting as one unit until the fly joins the velocity of the train, by splating spectacularly. As it splats it expends most of its energy in the z axis (looking at a side view) and some of its energy slows the train down. You're getting too wrapped up in thinking they're the same thing.

If you look at SteveC's drawing (seriously, you took the time to draw that?? ) you can't consider them one unit until the fly's velocity catches up with the train's - it's moving by deforming the train and itself.

I like.. this is what I said couple posts ago... the "during the decelleration of the fly..." thingy.
You should have seen the faces of non-technical major people when i asked a group of friends before... beautiful !!!! that "dear in the headlights" look you guys mentioned before somewhere.. lol!!!

 

[CaptBill]

No wonder Amtrak is always running late.

 

[TallFlyer]

So I copied this thread to an engineering student friend of mine and this is what he comes up with:

So I'll tell you what really happens. At the instant of collision, the two objects (the plane and the fly) the computer that models the world performs a calculation to see which one "wins" the collision. During the time expended for that calculation, the fly and the train are both at v = 0, but this cannot be directly observed because to cover for this glitch (or bug if you will) the entire "Earth" program pauses until the calculation has finished. Because of the overwhelming complexity of the "Earth" program, the actual time elapsed from the perspective of an outside observer is on the order of 1 second "Earth" time to 1 week "Observer" time, which is why it probably feels like an eternity since you started reading this post...

Now, if I may point something out, if the train is only going 30 MPH and the fly in question is your average housefly he'd be much more likely to bounce off than splatter. Of course, if the train in question is a French TGV then certainly his life is in peril. Just a thought. Granted, I have no idea how that affects the question, although CF makes the most sense to me, but yeah... I just fly airplanes, what do I know?