Does the fly stop the train.

gevo

gevo

New Member
Well, once you read through this little scenerio, tell me where the leap of logic is.

You have a locomotive travelling 30mph on a straight portion of track. You have a fly, travelling at whatever speed fly's travel at, directly towards the plane. It is imoprtant to understand the fly and the train are in exactly opposing directions, where their vectoral directions are parallel. When the fly hits the train the fly changes direction, this is obvious no secret here. At some point when an object changes direction, it has a velocity of zero. At the point the fly changes direction, it is also travelling 0mph. Now, since the fly changes direction BECAUSE it hits the train, and it being a basic understanding of physics that when something is attached to another object, they can't be going different velocities, atleast not in our worlds' 3 dimensions. Does this not mean that there is some point along the line, where the fly is travelling 0mph, and it is attached to the train which then must also travel 0mph...?

Sure, we have coefficient of restitution of the fly.. but regardless, every particle of the fly once it is splattered still must change direction.

Now, the obvious thing is that the train DOESN"T stop.. if one was to argue that it just stops for such a short time you don't notice it.. well, then the accelleration would be too massive for the train to decellerate and then accellerate back to 30mph in such a short time..

where is the leap of logic? I'm not saying there IS one for sure either :-D
 
gevo said:
Well, once you read through this little scenerio, tell me where the leap of logic is.

You have a locomotive travelling 30mph on a straight portion of track. You have a fly, travelling at whatever speed fly's travel at, directly towards the plane. It is imoprtant to understand the fly and the train are in exactly opposing directions, where their vectoral directions are parallel. When the fly hits the train the fly changes direction, this is obvious no secret here. At some point when an object changes direction, it has a velocity of zero. At the point the fly changes direction, it is also travelling 0mph. Now, since the fly changes direction BECAUSE it hits the train, and it being a basic understanding of physics that when something is attached to another object, they can't be going different velocities, atleast not in our worlds' 3 dimensions. Does this not mean that there is some point along the line, where the fly is travelling 0mph, and it is attached to the train which then must also travel 0mph...?

Sure, we have coefficient of restitution of the fly.. but regardless, every particle of the fly once it is splattered still must change direction.

Now, the obvious thing is that the train DOESN"T stop.. if one was to argue that it just stops for such a short time you don't notice it.. well, then the accelleration would be too massive for the train to decellerate and then accellerate back to 30mph in such a short time..

where is the leap of logic? I'm not saying there IS one for sure either :-D
Click to expand...

Please do not drive tonight. :buck:
 
I hit a really gross green bug with my truck today. It went *splat*. Is that kind of what happens to the fly in your story?
 
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.
 
What happened to the plane in your first sentence?:confused:

If you were traveling at the speed of light and looked in a mirror would you see yourself?:buck:
 
gevo said:
Well, once you read through this little scenerio, tell me where the leap of logic is.

You have a locomotive travelling 30mph on a straight portion of track. You have a fly, travelling at whatever speed fly's travel at, directly towards the plane. It is imoprtant to understand the fly and the train are in exactly opposing directions, where their vectoral directions are parallel. When the fly hits the train the fly changes direction, this is obvious no secret here. At some point when an object changes direction, it has a velocity of zero. At the point the fly changes direction, it is also travelling 0mph. Now, since the fly changes direction BECAUSE it hits the train, and it being a basic understanding of physics that when something is attached to another object, they can't be going different velocities, atleast not in our worlds' 3 dimensions. Does this not mean that there is some point along the line, where the fly is travelling 0mph, and it is attached to the train which then must also travel 0mph...?

Sure, we have coefficient of restitution of the fly.. but regardless, every particle of the fly once it is splattered still must change direction.

Now, the obvious thing is that the train DOESN"T stop.. if one was to argue that it just stops for such a short time you don't notice it.. well, then the accelleration would be too massive for the train to decellerate and then accellerate back to 30mph in such a short time..

where is the leap of logic? I'm not saying there IS one for sure either :-D
Click to expand...


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.
 
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.
 
Constant momentum collision with a coefficient of restitution of 0 (they stick).

You really can't model collisions in the way that you did. Objects don't instantly "stick", plus all the forces act over a (very short) period of time called the impulse.

I wish I could explain it better, but I don't think I can at 1130 at night.
 
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.
 
pullup said:
Didn't say the train stopped for a nanosecond, I said that the relative velocities of each were 0 for a nanosecond...it's all in the frame of reference.
Click to expand...


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 longer than a nanosecond. :)
 
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