If I am standing on a high speed train standing in the corridor, if I jump in the air, will I land in the same place carried by the motion of the train, or will I land a step or two behind, given that the train will continue moving whereas I am no longer being propelled forward?
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TRIVIA: a question for physicists
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433N
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In all likelihood, in the same place.
Whilst you may think you are jumping 'up' what you are doing is imparting a vertical component to your velocity - you still have the same horizontal component to your velocity as when you are standing still and so you will carry on at the same horizontal velocity as before you jumped.
[Pedantically, I could bore you with Einstein's theory of General Relativity and intertial frames but you probably wouldn't thank me for it].
Whilst you may think you are jumping 'up' what you are doing is imparting a vertical component to your velocity - you still have the same horizontal component to your velocity as when you are standing still and so you will carry on at the same horizontal velocity as before you jumped.
[Pedantically, I could bore you with Einstein's theory of General Relativity and intertial frames but you probably wouldn't thank me for it].
Dai Corner
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What do you mean by 'the same place'?
The same area of the train floor? Yes, roughly.
But that will itself have moved relative to the track.
The same area of the train floor? Yes, roughly.
But that will itself have moved relative to the track.
underbank
Established Member
You'll be in the same place due to your own personal momentum which would continue to propel you forward even if the train suddenly stopped. The air around you will also be moving at the same speed of the train, so there'll be a little friction from them helping to propel you forward too.
AlterEgo
Veteran Member
You will land slightly behind where you started due to air resistance. It’s the same resistance you experienced when standing on the train, but as you don’t have the friction of your feet to hold you steady, you can expect to move backwards slightly.
AlterEgo
Veteran Member
This doesn’t make sense. If your train is going 100mph forward, the air a metre above the roof of the train isn’t also going 100mph!
If the train suddenly stopped, wouldn't your momentum carry you further forward than the train?
underbank
Established Member
No, but the air inside the train, where you are, is travelling at 100mph
AlterEgo
Veteran Member
Ah, my apologies - misread the bit about being actually inside the train! Whoops!
edwin_m
Veteran Member
If the train is braking you'll land slightly further towards the front, and if it's accelerating you'll land slightly further towards the back. But since the braking and acceleration rates of trains are quite low then it's unlikely to be noticeable. A more extreme version is going through the windscreen of a car if it stops suddenly and you aren't wearing a seat belt.
But would I be right in saying that if you stayed in the air for a few seconds (with whatever magic) you would lose that forward momentum and the train would continue without you with disastrous effects on your body at the back of the coach ?
To put it even more in perspective, what would happen to a bee flying in the train ? If it suddenly stopped but kept itself in the air with its wings ?
Peter Mugridge
Veteran Member
No, because the air is being carried forwards at the same speed as the train. This assumes, of course, that you are in an enclosed carriage with full sides and a roof.
Similarly, the bee would hover in the same position within the carriage.
Yes that seems logical indeed.
Unless there is an open window, with some wind arriving in the bee’s face in which case it would be pushed backwards - but that would just be the result of the draught and not the forwards movement of the train.
Unless there is an open window, with some wind arriving in the bee’s face in which case it would be pushed backwards - but that would just be the result of the draught and not the forwards movement of the train.
DynamicSpirit
Established Member
The answer becomes very obvious if you frame the same question in terms of the Earth: The Earth orbits the sun at a rather astonishing 30 km/second: In other words, it takes the Earth less than 3 seconds to move the distance from London to Brighton.
If you are out in the countryside and you jump in the air - and, say, manage to stay up for 1 second - will you land in the same place, even though you are no longer being propelled forward? If the answer was that you'd land 30km behind where you jumped from, I doubt anyone would ever have bothered inventing trains (or cars, buses, bicycles or aeroplanes, for that matter)!
(One flaw in the question is that the use of the word 'propelled' is misleading. You are not being propelled on either a train or the Earth... you are simply obeying the laws of physics by not accelerating horizontally in the absence of any force causing you to accelerate horizontally).
That would be fun - always assuming you wanted to go in that direction! If not, you would either have to walk back, or go all round the world!
The answer is about force.
If the train is travelling at a constant velocity (in all 3 spatial dimensions), then regardless of how long you are magically suspended in mid air , then when you land it will be in the same place. This is because there is no net force being applied to you, or the train. (The force the train is exerting is being used to overcome resistance to motion, and st a constant velocity they exactly cancel each other out).
If, however, the train is accelerating or decelerating - again in any of the three spatial dimensions - then you would move during your magical suspension. Probably the best example is if you went around a corner: the train is being ‘accelerated’ to one side, but you would not be. You would then end up, fairly gently, in contact with the inside side of the train, at which point you would then start to experience the same acceleration as the train. You will experience this acceleration every time you go round a corner on a train, particularly if you are standing up and not holding on to a grab pole.
As others have said, the accelerations a train is capable of under normal circumstances are rather gentle, and the air in the coach also imparts a certain amount of resistance. Emergency braking (nominally 12%g) means that if you were travelling in a coach that was a perfect vacuum, and had a ‘hang time’ of 1 second, you would only move forward 0.6metres relative to the coach.
If the train is travelling at a constant velocity (in all 3 spatial dimensions), then regardless of how long you are magically suspended in mid air , then when you land it will be in the same place. This is because there is no net force being applied to you, or the train. (The force the train is exerting is being used to overcome resistance to motion, and st a constant velocity they exactly cancel each other out).
If, however, the train is accelerating or decelerating - again in any of the three spatial dimensions - then you would move during your magical suspension. Probably the best example is if you went around a corner: the train is being ‘accelerated’ to one side, but you would not be. You would then end up, fairly gently, in contact with the inside side of the train, at which point you would then start to experience the same acceleration as the train. You will experience this acceleration every time you go round a corner on a train, particularly if you are standing up and not holding on to a grab pole.
As others have said, the accelerations a train is capable of under normal circumstances are rather gentle, and the air in the coach also imparts a certain amount of resistance. Emergency braking (nominally 12%g) means that if you were travelling in a coach that was a perfect vacuum, and had a ‘hang time’ of 1 second, you would only move forward 0.6metres relative to the coach.
DerekC
Established Member
It all boils down to Newton's first law of motion - A body will remain at rest or continue with constant velocity in a straight line unless acted on by an external force. So when you are in the air inside the train, you continue to move with the same velocity as the train gave you when you took off. Assuming the train doesn't speed up or slow down while you are in the air you will land back at the same place on the train floor as you took off from.
krus_aragon
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And even though there are the forces of you pushing against the floor as you jump, and of gravity pulling you back down, if these are vertical then they won't affect your horizontal movement at all, because horizontal and vertical are perpendicular.
Strat-tastic
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Why don't you perform your own experiment? 
hooverboy
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really pedantically, no you will not be in the same place.
your velocity inside the train will remain basically the same, but the train has moved relative to the position of the earth, and the earth has moved relative to the position of the entire universe.
if you are talking scalar measurements then basic physics applies relative to just the train and the resultant forces on your body.
if you are talking vector measurements you're bloody miles away!
hence,why a broken clock IS NOT right twice a day!
DerekC
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really pedantically, no you will not be in the same place.
your velocity inside the train will remain basically the same, but the train has moved relative to the position of the earth, and the earth has moved relative to the position of the entire universe.
if you are talking scalar measurements then basic physics applies relative to just the train and the resultant forces on your body.
if you are talking vector measurements you're bloody miles away!
hence,why a broken clock IS NOT right twice a day!
All very relevant if you are steering a spacecraft - but (fortunately) not when you are inside a train! I think it's important when answering basic questions to make the answers simple and relevant to the context - otherwise people are inclined to think you are not consistent with common sense. And anyway (pedantically) the question was whether you and the train will be in the same relative position when you land as when you took off - and the answer (ignoring any relative motion of the air or acceleration or deceleration of the train) is yes. (Unless the train is going round a curve, of course).
edwin_m
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You think we're being pedantic? Nobody's mentioned relativistic effects yet... (oops).
Dai Corner
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I did use the word 'relative' in this post
I think the answer to the original question is probably something like 'it depends on your knowledge of physics, how deeply you analyse the scenario and what assumptions you make'
433N
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There are lots of answers to the original question because the question didn't specify conditions exactly. Since it was a question for 'physicists', I took it that it was posed by a layperson who wanted an answer to a first order approximation. If more accuracy was required then more information regarding conditions would be needed and certainly more would be required if we are to take into account relativistic corrections.
Being pedantic, we should also include the finite possibility that the person jumping passes through the floor on their descent but being pedantic is what gives physicists a bad name.
ATW Alex 101
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DynamicSpirit
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Haha! That did cross my mind when @krus_aragon referred to gravity 'pulling you back down' but I decided that would be just a little too pedantic for this thread
I don't want to look a wally!!!Try it
DerekC
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No it isn't - the question was very clear
And the answer, shorn of all the pseudo-scientific woffle, is "in the same place".
Dai Corner
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