The Physics Of Jumping Whilst On A Train

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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Tue Oct 06, 2009 7:24 pm

FatTrucker wrote:In a vaccuum where there is virtually no friction you just wouldn't slow down at all, moving forward forever at a constant velocity...
You wouldn't need a vaccuum to achieve that, surely? All you'd need is to have the windows on the train closed. eg: The carriage is moving at 70mph, you're standing in the carriage and therefore also moving at 70mph, the air in the carriage is moving at 70mph. If you could jump in the air and hover, you're still going to be moving forward at 70mph because there's nothing to stop you (except for maybe a teeny tiny little bit of air movement coming through the ventilation system). Open a few windows, and the moving air will slowly push you back.

For the those expecting our train jumper to go backwards down the carriage when he jumps in the air: that would only occur if the train were accelerating. eg: If you were on a train accelerating at, say, 3 metres per second², and you jumped in the air, then yes, from the pov of someone next to you on the carriage, you would appear to accelerate backward at something approaching 3 metres per second² (it would be less than 3 m/s² as the air in the carriage is also accelerating forward with the movement of the carriage to a certain degree, depending on how many windows are open, etc)

Just think of the pressure you feel when sitting in a car that is rapidly accelerating - as the car accelerates, you feel as if you're being pushed back in your seat a little. Once the car stops accelerating and reaches a steady velocity, you don't feel as if you're being pushed back at all - instead you might as well be sitting in a normal stationary seat (assuming the car is travelling in a straight line). Then if you rapidly decelerate, you feel as if you're being thrown forward against your seatbelt. Those forces when accelerating/decelerating and the lack of back/forward forces when travelling at a constant velocity tell you pretty much all you need to know about jumping while on a train (at least as far as movement in a straight line goes).
Last edited by Emperor Fossil on Tue Oct 06, 2009 9:02 pm, edited 1 time in total.

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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Tue Oct 06, 2009 7:36 pm

I'm not sure that it can be said that the air particles have a forward velocity equal to that of the train. After all, they are free moving and therefore moving at every conceivable angle, so very few particles will in fact have a forward velocity matching that of the train. There are also convenction currents and Brownian motion to think about, and that's before we even consider things like open windows!
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Re: The Physics Of Jumping Whilst On A Train

Post by The Master » Tue Oct 06, 2009 7:36 pm

Sometimes..... sometimes I really think I should just keep my big mouth shut :lol:
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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Tue Oct 06, 2009 7:37 pm

It's much more fun if you don't!
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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Tue Oct 06, 2009 7:46 pm

Havantgottaclue wrote:I'm not sure that it can be said that the air particles have a forward velocity equal to that of the train. After all, they are free moving and therefore moving at every conceivable angle, so very few particles will in fact have a forward velocity matching that of the train. There are also convenction currents and Brownian motion to think about, and that's before we even consider things like open windows!
But as a mass, it's generally moving at the same speed of the train. Convection currents and Brownian motion are pretty much negligible in this scenario. You have the same effects in the room you're in right now, but if you could jump in the air and hover, would you expect them to be pushing you around to any noticeable degree? (consider the mass of your body versus the minute nature of those forces)

Or to put it another way, when you're sitting there in the train moving at a constant velocity with the windows closed, do you feel any wind or air movement on your face? To all intents and purposes, you might as well be sitting in a stationary carriage at that point.

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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Tue Oct 06, 2009 7:55 pm

Yes, I think I can see your point! :)
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Re: The Physics Of Jumping Whilst On A Train

Post by markopoloman » Tue Oct 06, 2009 8:02 pm

I have just got in from work. This is not what I expect to find on the RG forum! Could we PLEASE do something a little more simple in future?

Master - do the Tea or Coffee thing next time :shock:
OFF TOPIC DISCUSSIONS - http://retrocanteen.boards.net/

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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Tue Oct 06, 2009 8:06 pm

Now all we need is something like the 'consider a bucket of water with cork in it, but the cork is attached to a spring that is attached to the bottom of the bucket and is keeping the cork just under the water level, and then the bucket is dropped off a building' question.

Or maybe just the dreaded 'jet plane on a conveyor belt' question. ;)

markopoloman wrote:Master - do the Tea or Coffee thing next time :shock:
What if we switched it around and asked "Coffee or Tea?"

Could make a nice change.
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Re: The Physics Of Jumping Whilst On A Train

Post by The Master » Tue Oct 06, 2009 8:08 pm

What? Let's just settle for the Marmite conundrum :lol:
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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Wed Oct 07, 2009 3:26 am

markopoloman wrote:I have just got in from work. This is not what I expect to find on the RG forum! Could we PLEASE do something a little more simple in future?

Master - do the Tea or Coffee thing next time :shock:
How about this then: anyone know why snot is green?
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Re: The Physics Of Jumping Whilst On A Train

Post by mohicankid » Wed Oct 07, 2009 3:40 am

The Master wrote:Considering that there's no currently accepted valid scientific explanation for how the fuck Bumblebees can actually fly, I don't think my question was too retarded :P

If I fall off, I'll never speak to you all again :twisted:
just don't jump sidways!

i remember asking a similar question in school when i was 8, but it involved jumping in an elevater not a train (why don't i bash my head if the lifts going down?) and got the speed velocity answer!
Just when you thought it was safe to go back in the water...

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Re: The Physics Of Jumping Whilst On A Train

Post by FatTrucker » Wed Oct 07, 2009 3:50 am

Emperor Fossil wrote:
FatTrucker wrote:In a vaccuum where there is virtually no friction you just wouldn't slow down at all, moving forward forever at a constant velocity...
You wouldn't need a vaccuum to achieve that, surely? All you'd need is to have the windows on the train closed. eg: The carriage is moving at 70mph, you're standing in the carriage and therefore also moving at 70mph, the air in the carriage is moving at 70mph. If you could jump in the air and hover, you're still going to be moving forward at 70mph because there's nothing to stop you (except for maybe a teeny tiny little bit of air movement coming through the ventilation system). Open a few windows, and the moving air will slowly push you back.

For the those expecting our train jumper to go backwards down the carriage when he jumps in the air: that would only occur if the train were accelerating. eg: If you were on a train accelerating at, say, 3 metres per second², and you jumped in the air, then yes, from the pov of someone next to you on the carriage, you would appear to accelerate backward at something approaching 3 metres per second² (it would be less than 3 m/s² as the air in the carriage is also accelerating forward with the movement of the carriage to a certain degree, depending on how many windows are open, etc)

Just think of the pressure you feel when sitting in a car that is rapidly accelerating - as the car accelerates, you feel as if you're being pushed back in your seat a little. Once the car stops accelerating and reaches a steady velocity, you don't feel as if you're being pushed back at all - instead you might as well be sitting in a normal stationary seat (assuming the car is travelling in a straight line). Then if you rapidly decelerate, you feel as if you're being thrown forward against your seatbelt. Those forces when accelerating/decelerating and the lack of back/forward forces when travelling at a constant velocity tell you pretty much all you need to know about jumping while on a train (at least as far as movement in a straight line goes).
There's still friction, just not a lot which is why you would decelerate slowly and not just shoot straight to the back and why you don't move at all if you jump but you would slowly drift backward if you floated.
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Re: The Physics Of Jumping Whilst On A Train

Post by FatTrucker » Wed Oct 07, 2009 3:54 am

The Master wrote:
FatTrucker wrote:When you think about it, if all of reality was a void with no other matter than yourself, no friction or gravity to affect movement, how would you define movement, velocity etc?. Would you actually be moving at all, if there were no relative objects by which to measure speed and nothing to get closer to and further away from?
This, surely, is a bit like the perennial "if a tree fell down in the forest and there was nobody there to hear it, would it still make a noise?" question.

My answer to this one, as always, is a resounding "of course it fucking would - LOL" :lol:
But we define movement and speed based on relativity and proximity, how would you be moving if you're in an infinite void of nothing?, how would you even know if you're moving?....absurd premise but interesting nonetheless.
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Re: The Physics Of Jumping Whilst On A Train

Post by Emperor Fossil » Wed Oct 07, 2009 4:10 am

FatTrucker wrote:
Emperor Fossil wrote:
FatTrucker wrote:In a vaccuum where there is virtually no friction you just wouldn't slow down at all, moving forward forever at a constant velocity...
You wouldn't need a vaccuum to achieve that, surely? All you'd need is to have the windows on the train closed. eg: The carriage is moving at 70mph, you're standing in the carriage and therefore also moving at 70mph, the air in the carriage is moving at 70mph. If you could jump in the air and hover, you're still going to be moving forward at 70mph because there's nothing to stop you (except for maybe a teeny tiny little bit of air movement coming through the ventilation system). Open a few windows, and the moving air will slowly push you back.
There's still friction, just not a lot which is why you would decelerate slowly and not just shoot straight to the back and why you don't move at all if you jump but you would slowly drift backward if you floated.
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.)
Havantgottaclue wrote:How about this then: anyone know why snot is green?
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Re: The Physics Of Jumping Whilst On A Train

Post by Havantgottaclue » Wed Oct 07, 2009 4:31 am

The only thing I could find on jumping on trains on teh interwebz was on a physics forum imaginatively titled ... um ... Physics Forum. Not that the answers are any more eloquent than the ones given by EF and FT here ...
Emperor Fossil wrote:Myeloperoxidase :mrgreen: <- seemed an appropriate smiley.
Ahh, so it's the iron content of the enzyme used to clear the infection ... interesting.
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