The Physics Of Jumping Whilst On A Train

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Re: The Physics Of Jumping Whilst On A Train

Post by Commander Jameson » Wed Oct 07, 2009 5:21 am

I personally prefer coffee.



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Re: The Physics Of Jumping Whilst On A Train

Post by FatTrucker » Wed Oct 07, 2009 7:37 am

Emperor Fossil wrote: But why would you decelerate? What force is acting upon you to decrease your speed? I think you're making the mistake of assuming that the air will be pushing you back, but the air in the carriage is moving with the carriage at the same speed. (remember, we have the windows shut in this example.)
There is no 'force' acting to 'push' you back. There are air particles moving about though, which will create friction and drag and slow you down without a physical connection to the train you're moving in. The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag. If you smoke inside a moving train with the windows shut the smoke will also slowly drift toward the back of the train.
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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Wed Oct 07, 2009 8:28 am

FatTrucker wrote:
Emperor Fossil wrote: But why would you decelerate? What force is acting upon you to decrease your speed? I think you're making the mistake of assuming that the air will be pushing you back, but the air in the carriage is moving with the carriage at the same speed. (remember, we have the windows shut in this example.)
There is no 'force' acting to 'push' you back. There are air particles moving about though, which will create friction and drag and slow you down without a physical connection to the train you're moving in. The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag. If you smoke inside a moving train with the windows shut the smoke will also slowly drift toward the back of the train.
That makes sense in the real world, because no train is completely air tight. Whatever air comes through cracks and open windows would reduce the overall forward velocity of the air mass by a percentage. But I think I get what EF is saying now - if the container is perfectly sealed, the air particles are all moving at different velocities but they all cancel each other out, the overall effect being that the air mass as a whole is moving at the same speed as the container, and so the drag or friction they apply would be the same coming from every direction. So, if a flying insect gets into a train, it doesn't have to make any effort to keep up with the train, aside from countering reductions in the air mass speed through leakage, which of course would be much worse by a window - a bee couldn't possibly get out of a train window while it was moving, I guess.

I'm trying to get it to make sense to myself by thinking about this example: if you threw an airtight box full of smoke particles, the smoke would end up uniformly dispersed in the box if the box reached a constant velocity. If the box accellerates, the smoke particles move to the back, if it decelerates, they move to the front. The same applies to any body hovering inside the box, regardless of the size or shape, assuming that at some point it received all the propulsive energy the actual box did (for example by standing on the floor of the box and then jumping when a constant speed has been reached).

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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Wed Oct 07, 2009 8:33 am

FatTrucker wrote:
Emperor Fossil wrote: But why would you decelerate? What force is acting upon you to decrease your speed? I think you're making the mistake of assuming that the air will be pushing you back, but the air in the carriage is moving with the carriage at the same speed. (remember, we have the windows shut in this example.)
There is no 'force' acting to 'push' you back. There are air particles moving about though, which will create friction and drag and slow you down without a physical connection to the train you're moving in. The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag. If you smoke inside a moving train with the windows shut the smoke will also slowly drift toward the back of the train.
If your velocity is going to decrease, as you're claiming it will, then there has to be a force acting upon you to cause that to happen - in other words, there must be a force that will cause you to decelerate, otherwise you simply won't decelerate. That's Newton's 1st law right there. For example, If you were to jump out of the moving train, you'd hit the still air outside, and the resistance of that air against your moving body would be the force that causes deceleration. (And then if you were unlucky you'd hit a tree, which would offer much firmer resistance and hence a much more effective opposing force, and you'd decelerate very suddenly and painfully.)

So in this case, the force that you're claiming will cause the person to decelerate is exerted by the air in the carriage, which you're saying is going to be moving slowly back towards the rear of the carriage (from the pov of someone on the train), hence your claim about the smoke drifting to the rear. Basically, you seem to be suggesting that the air in the carriage will be travelling at a slower velocity than the carriage itself.
The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag.
But why won't the air be moving at the same constant velocity as the train? To clarify, if the carriage was air-tight (not a vacuum, just air-tight), would you still claim that the smoke in the air would drift towards the rear?

What I'm claiming is that if the train is indeed travelling at a constant velocity, then the smoke is only going to drift towards the rear if there's a ventilation system allowing or directing airflow from front to back. If the vents are just letting air filter in gently from the sides, I doubt you'd get much, if any, of the effect you describe. And if the carriage was air-tight, then you certainly wouldn't get the effect you describe. Not unless the train was actually accelerating, of course.

edit: yep, HGAC gets what I'm trying to say. Another somewhat similar example is this:

Fill a glass with water, then find a nice long stretch of road and start walking briskly in a straight line. As you accelerate to your brisk walk speed, the water will push up against one side of the glass (in the opposite direction to the acceleration), but once you reach your brisk walk speed and stop accelerating, you'll find that the water levels out again (assuming you can keep your speed constant and your hand fairly steady). Decelerate, and it will push up against the other side of the glass. It's the same principle as the air in the carriage.

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Re: The Physics Of Jumping Whilst On A Train

Post by madblokeything » Wed Oct 07, 2009 9:15 am

but.....noboby can hear your screams in space.........so if you jump up in space your fucked...so stay on the train ...:)
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Re: The Physics Of Jumping Whilst On A Train

Post by pantal00ns » Wed Oct 07, 2009 9:16 am

$$£^^(%$JGFR{)"("+&_%($NSfiern4e9££($~4wpih-ef.......

SYNTAX ERROR.....

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Simply consider this...

Would you kill someone for 1Million pounds by jumping off a train with tea or coffee?

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Re: The Physics Of Jumping Whilst On A Train

Post by FatTrucker » Wed Oct 07, 2009 11:24 am

Emperor Fossil wrote:
FatTrucker wrote:
Emperor Fossil wrote: But why would you decelerate? What force is acting upon you to decrease your speed? I think you're making the mistake of assuming that the air will be pushing you back, but the air in the carriage is moving with the carriage at the same speed. (remember, we have the windows shut in this example.)
There is no 'force' acting to 'push' you back. There are air particles moving about though, which will create friction and drag and slow you down without a physical connection to the train you're moving in. The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag. If you smoke inside a moving train with the windows shut the smoke will also slowly drift toward the back of the train.
If your velocity is going to decrease, as you're claiming it will, then there has to be a force acting upon you to cause that to happen - in other words, there must be a force that will cause you to decelerate, otherwise you simply won't decelerate. That's Newton's 1st law right there. For example, If you were to jump out of the moving train, you'd hit the still air outside, and the resistance of that air against your moving body would be the force that causes deceleration. (And then if you were unlucky you'd hit a tree, which would offer much firmer resistance and hence a much more effective opposing force, and you'd decelerate very suddenly and painfully.)

So in this case, the force that you're claiming will cause the person to decelerate is exerted by the air in the carriage, which you're saying is going to be moving slowly back towards the rear of the carriage (from the pov of someone on the train), hence your claim about the smoke drifting to the rear. Basically, you seem to be suggesting that the air in the carriage will be travelling at a slower velocity than the carriage itself.
The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag.
But why won't the air be moving at the same constant velocity as the train? To clarify, if the carriage was air-tight (not a vacuum, just air-tight), would you still claim that the smoke in the air would drift towards the rear?

What I'm claiming is that if the train is indeed travelling at a constant velocity, then the smoke is only going to drift towards the rear if there's a ventilation system allowing or directing airflow from front to back. If the vents are just letting air filter in gently from the sides, I doubt you'd get much, if any, of the effect you describe. And if the carriage was air-tight, then you certainly wouldn't get the effect you describe. Not unless the train was actually accelerating, of course.

edit: yep, HGAC gets what I'm trying to say. Another somewhat similar example is this:

Fill a glass with water, then find a nice long stretch of road and start walking briskly in a straight line. As you accelerate to your brisk walk speed, the water will push up against one side of the glass (in the opposite direction to the acceleration), but once you reach your brisk walk speed and stop accelerating, you'll find that the water levels out again (assuming you can keep your speed constant and your hand fairly steady). Decelerate, and it will push up against the other side of the glass. It's the same principle as the air in the carriage.
Because most of the air inside the carriage isn't actually in contact with the carriage most of the time.
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Re: The Physics Of Jumping Whilst On A Train

Post by TwoHeadedBoy » Wed Oct 07, 2009 2:53 pm

pantal00ns wrote: Simply consider this...

Would you kill someone for 1Million pounds by jumping off a train with tea or coffee?
Erm... Mega Drive?
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Re: The Physics Of Jumping Whilst On A Train

Post by Scapegoat » Wed Oct 07, 2009 4:29 pm

The Master wrote:Well how come when I try to replicate this experiment with a little model choo-choo train and a LEGO man, he always falls off? :?:

The answer to that is simple, the wind resistance of exterior to a toy train is great enough for a light plastic toy to be affected and slowed down enough to fall off..

If you were on a train travelling at 120 mph and tossed the same lego man in the air he'd come straight down again, but if you were in an open top carriage on the same train, he's be blown away.
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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Wed Oct 07, 2009 5:25 pm

FatTrucker wrote:
Emperor Fossil wrote:
The air inside the train will not be moving at exactly the same constant velocity as the train, which will create friction, which will create drag.
But why won't the air be moving at the same constant velocity as the train? To clarify, if the carriage was air-tight (not a vacuum, just air-tight), would you still claim that the smoke in the air would drift towards the rear?
Because most of the air inside the carriage isn't actually in contact with the carriage most of the time.
That didn't really answer my question about if the carriage was air-tight, would you still claim the smoke in the air would drift towards the rear... but it doesn't matter. Basically, you're wrong. ;) You seem to be treating it as if the train is accelerating, rather than travelling at a constant velocity. I know it seems counter-intuitive to some extent, but when the train is travelling along at a constant velocity with the windows shut (and let's block off the vents for good measure), then that volume of air in the carriage will be travelling at the same speed as the carriage itself. In the same manner, if you held a glass jar in your hands and sat in a trolley and had someone push you at a constant velocity along a flat road, the air in that jar would be moving at the same speed as the jar itself, filling the volume of the jar at even pressure.

Anyway, I'm not going to harp on this, as if I do I'll surely earn the title of the nerdiest nerd on the forum. If I haven't already.

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Re: The Physics Of Jumping Whilst On A Train

Post by bonerlaw » Sun Oct 11, 2009 3:44 pm

Just flown back from Edingburgh, and i have a similar sort of queary to the topic title!

Why when the plane was turning do liquids stay upright? Surely if the plane is tilting to one side, the water in the bottle should do likewise? :?
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Re: The Physics Of Jumping Whilst On A Train

Post by Antiriad2097 » Sun Oct 11, 2009 4:35 pm

It does. Its just subtle enough you didn't notice. There will be some small effect from the g forces of the plane itself accelerating sideways that will reduce it further, but for the most part your liquids behave 'normally' - tilt the plane, which tilts the bottle, and your liquid surfaces will remain mostly in the horizontal.
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Re: The Physics Of Jumping Whilst On A Train

Post by paranoid marvin » Sun Oct 11, 2009 5:33 pm

The most importan t question of course is , was this taken into account by Hudsonsoft when they programmed 'Stop The Express'?
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Re: The Physics Of Jumping Whilst On A Train

Post by kebabage » Sun Oct 11, 2009 5:39 pm

The Physics of taking a dump on a train...

..Would you make skids every time?
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Re: The Physics Of Jumping Whilst On A Train

Post by paranoid marvin » Sun Oct 11, 2009 5:43 pm

kebabage wrote:The Physics of taking a dump on a train...

..Would you make skids every time?

Is that where Brownian Motion comes into effect?
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