Things you always wanted to know the answer to [Vol. 3]
Discussion
RobinOakapple said:
In physics, AIUI, the classic definition of work is weight times height. It was easily measured and not dependant on time. So the person going uphill is working harder than the person who isn't.
Height, in this case, being the distance above wherever you started.On both elevators you end up the same distance above the place you started. It matters not whether you are going up or the floor is going down.
Instead of gaining potential energy (walking up a stationary staircase) you have avoided losing potential energy (being carried down by an escalator).
The net result is the same in both cases. (minor nit-picky variations excepted.)
SpeckledJim said:
RobinOakapple said:
In physics, AIUI, the classic definition of work is weight times height. It was easily measured and not dependant on time. So the person going uphill is working harder than the person who isn't.
Height, in this case, being the distance above wherever you started.On both elevators you end up the same distance above the place you started. It matters not whether you are going up or the floor is going down.
Instead of gaining potential energy (walking up a stationary staircase) you have avoided losing potential energy (being carried down by an escalator).
The net result is the same in both cases. (minor nit-picky variations excepted.)
The idea of not losing something being the same as gaining it is interesting but simply doesn't apply here.
The person's upper body is staying mostly still, the downward motion of the escalator is accommodated by his moving his legs in time with it. So his legs are moving but (in effect) his trunk and arms are not. The point about not losing height simply doesn't apply. Therefore the net result is not the same, quite a bit more effort to climb a stationary escalator than to stay in one place on a down escalator.
Rostfritt said:
Apparently in a safe on one of our submarines there is a letter penned by the serving PM with instructions of what to do if the British government has fallen. One of the checks they make before they open the letter is if Radio 4 is still broadcasting.
I wonder what happens to the old letters? Did they flush the one Gordon Brown wrote down the bog or is it in a box somewhere?Timmy40 said:
Rostfritt said:
Apparently in a safe on one of our submarines there is a letter penned by the serving PM with instructions of what to do if the British government has fallen. One of the checks they make before they open the letter is if Radio 4 is still broadcasting.
I wonder what happens to the old letters? Did they flush the one Gordon Brown wrote down the bog or is it in a box somewhere?moustachebandit said:
dudleybloke said:
marshalla said:
moustachebandit said:
During WW2 we had warning systems to let us know of an impending attack. I was just wondering do we have something like that still in place now?
So if Aliens decided to attack will air raid sirens go off so we can all run for cover or will the first notification we get about it beon the TV in the Sean Connery Joke thread ?
We no longer have a network of sirens, but the BBC have the capability to broadcast in emergencies.So if Aliens decided to attack will air raid sirens go off so we can all run for cover or will the first notification we get about it be
Mind you - if the aliens are intent on taking over, it's unlikely that anyone would have much time to do anything.
I, for one, welcome our new formless overlords.
Edited by marshalla on Monday 18th April 14:11
I think mobile phones would be used.
Link
RobinOakapple said:
SpeckledJim said:
RobinOakapple said:
In physics, AIUI, the classic definition of work is weight times height. It was easily measured and not dependant on time. So the person going uphill is working harder than the person who isn't.
Height, in this case, being the distance above wherever you started.On both elevators you end up the same distance above the place you started. It matters not whether you are going up or the floor is going down.
Instead of gaining potential energy (walking up a stationary staircase) you have avoided losing potential energy (being carried down by an escalator).
The net result is the same in both cases. (minor nit-picky variations excepted.)
The idea of not losing something being the same as gaining it is interesting but simply doesn't apply here.
The person's upper body is staying mostly still, the downward motion of the escalator is accommodated by his moving his legs in time with it. So his legs are moving but (in effect) his trunk and arms are not. The point about not losing height simply doesn't apply. Therefore the net result is not the same, quite a bit more effort to climb a stationary escalator than to stay in one place on a down escalator.
Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
SpeckledJim said:
But the entire body on the moving staircase is n metres higher than it would have been had the legs not done the work.
Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
Yes, but simple physics is not concerned about what would have happened if something else hadn't happened, it's concerned with things that do happen. And in the case of the person on the down elevator, the legs move but the upper body doesn't. What the mass of the legs is doing is no concern of the mass of the upper body.Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
RobinOakapple said:
SpeckledJim said:
But the entire body on the moving staircase is n metres higher than it would have been had the legs not done the work.
Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
Yes, but simple physics is not concerned about what would have happened if something else hadn't happened, it's concerned with things that do happen. And in the case of the person on the down elevator, the legs move but the upper body doesn't. What the mass of the legs is doing is no concern of the mass of the upper body.Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
So there isn't more gravity acting on an ascending figure than on a stationary figure.
Surely the principle of conservation of momentum means that both systems are doing the same work?
A figure moving steadily upwards is doing the same work as a figure maintaining position in a 'world' that is moving downwards. (air resistance notwithstanding).
An aeroplane in a headwind that is effectively 'hovering' above the ground is doing the same work as that same plane moving forwards through steady air, isn't it?
SpeckledJim said:
RobinOakapple said:
SpeckledJim said:
But the entire body on the moving staircase is n metres higher than it would have been had the legs not done the work.
Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
Yes, but simple physics is not concerned about what would have happened if something else hadn't happened, it's concerned with things that do happen. And in the case of the person on the down elevator, the legs move but the upper body doesn't. What the mass of the legs is doing is no concern of the mass of the upper body.Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
So there isn't more gravity acting on an ascending figure than on a stationary figure.
Surely the principle of conservation of momentum means that both systems are doing the same work?
A figure moving steadily upwards is doing the same work as a figure maintaining position in a 'world' that is moving downwards. (air resistance notwithstanding).
An aeroplane in a headwind that is effectively 'hovering' above the ground is doing the same work as that same plane moving forwards through steady air, isn't it?
I know I am pretty much repeating myself, but can't think of a better way to explain it.
RobinOakapple said:
SpeckledJim said:
RobinOakapple said:
SpeckledJim said:
But the entire body on the moving staircase is n metres higher than it would have been had the legs not done the work.
Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
Yes, but simple physics is not concerned about what would have happened if something else hadn't happened, it's concerned with things that do happen. And in the case of the person on the down elevator, the legs move but the upper body doesn't. What the mass of the legs is doing is no concern of the mass of the upper body.Just as the body on the stationary stairs is n metres higher, had the legs not done the work.
So there isn't more gravity acting on an ascending figure than on a stationary figure.
Surely the principle of conservation of momentum means that both systems are doing the same work?
A figure moving steadily upwards is doing the same work as a figure maintaining position in a 'world' that is moving downwards. (air resistance notwithstanding).
An aeroplane in a headwind that is effectively 'hovering' above the ground is doing the same work as that same plane moving forwards through steady air, isn't it?
I know I am pretty much repeating myself, but can't think of a better way to explain it.
Once a body is moving, (in a vacuum, outside gravity) it requires no force at all to keep it moving (Newton's First Law of Motion).
So, an initial bit of energy is required to get the body moving upwards (up the stairs). This is the propulsive force. Equally, in the other case an initial bit of energy is also required to stop the downwards travel and 'hold station' on the moving escalator. Also a propulsive force.
Thereafter, the inertia of the body (either moving relative to the earth or not) is equal, steady, and requires no further force to perpetuate it.
The only extra work is that required of the legs, to repel gravity and keep pace with the ground moving underneath the body. Which is moving at the same rate in both cases.
As far as I can see, the energies in and out match in both cases. Counter-intuitively, but I can't see where there's a mismatch of energies.
I think what you are mistaken about is you think it takes more energy to beat gravity when you are moving than when you are still, which is not correct.
Edited by SpeckledJim on Wednesday 20th April 12:53
SpeckledJim said:
So, an initial bit of energy is required to get the body moving upwards (up the stairs). This is the propulsive force.
Thereafter, the inertia of the body (either moving relative to the earth or not) is equal, steady, and requires no further force to perpetuate it.
I am guessing you haven't actually walked up stairs, here on earth?? Thereafter, the inertia of the body (either moving relative to the earth or not) is equal, steady, and requires no further force to perpetuate it.
Sadly you don't just set off up the first step and then conserve momentum all the way up.
Also your physics is wrong either way.
You can keep going PERPENDICULAR to the gravitational force (e.g. a billiard ball on a flat table) but going up stairs you are working at an angle to gravity and hence need to do some more work!
walm said:
SpeckledJim said:
So, an initial bit of energy is required to get the body moving upwards (up the stairs). This is the propulsive force.
Thereafter, the inertia of the body (either moving relative to the earth or not) is equal, steady, and requires no further force to perpetuate it.
I am guessing you haven't actually walked up stairs, here on earth?? Thereafter, the inertia of the body (either moving relative to the earth or not) is equal, steady, and requires no further force to perpetuate it.
Sadly you don't just set off up the first step and then conserve momentum all the way up.
Also your physics is wrong either way.
You can keep going PERPENDICULAR to the gravitational force (e.g. a billiard ball on a flat table) but going up stairs you are working at an angle to gravity and hence need to do some more work!
OK, consider my aeroplane in a headwind analogy. Is it doing less work flying through a 100mph headwind and hovering above the ground than it is flying forwards at 100mph through still air?
walm said:
SpeckledJim said:
As far as I can see, the energies in and out match in both cases.
In one case, you gain height.In the other, you can start in the middle and end up in the same place. No height gain.
At the moment when the space shuttle takes off and is climbing at 0.0001 mph, is it expending less energy than when it is climbing at 1000 mph?
The escalator question has the legs responsible for 'thrust'. And it's the same in both cases.
SpeckledJim said:
Your point assumes that climbing is working against gravity, but hoving isn't. Gravity is the same in both cases.
OK, consider my aeroplane in a headwind analogy. Is it doing less work flying through a 100mph headwind and hovering above the ground than it is flying forwards at 100mph through still air?
Yes. Climbing is working against gravity.OK, consider my aeroplane in a headwind analogy. Is it doing less work flying through a 100mph headwind and hovering above the ground than it is flying forwards at 100mph through still air?
(You have more potential energy at the top of some stairs than at the bottom or middle.)
Your aeroplane is doing the same work, as the (unchanging) height is the same in both examples.
SpeckledJim said:
But the 'thrust' required for both is equal.
No. It isn't.For a start the mass is different.
Robin's example of splitting the body in two isn't perfect but helps.
To keep stationary on a moving escalator (and if you happen to move like Michael Jackson) you just need to move your legs, rather than your upper body (much).
Whereas to go from the bottom to the top you need to move your entire body.
walm said:
SpeckledJim said:
Your point assumes that climbing is working against gravity, but hoving isn't. Gravity is the same in both cases.
OK, consider my aeroplane in a headwind analogy. Is it doing less work flying through a 100mph headwind and hovering above the ground than it is flying forwards at 100mph through still air?
Yes. Climbing is working against gravity.OK, consider my aeroplane in a headwind analogy. Is it doing less work flying through a 100mph headwind and hovering above the ground than it is flying forwards at 100mph through still air?
(You have more potential energy at the top of some stairs than at the bottom or middle.)
Your aeroplane is doing the same work, as the (unchanging) height is the same in both examples.
A hovering rocket is doing the same work in a downdraft as a climbing rocket.
Now change the rocket for a pair of legs, and swap the downdraft for a moving staircase.
same:same?
What am I missing?
The potential energy gained by actually going upwards is balanced by the additional force put into the moving staircase by someone holding station on it. The staircase is being 'helped' downwards by the work done by your legs.
That's a force equal to the one that pushes you upwards on a static staircase.
walm said:
SpeckledJim said:
But the 'thrust' required for both is equal.
No. It isn't.For a start the mass is different.
Robin's example of splitting the body in two isn't perfect but helps.
To keep stationary on a moving escalator (and if you happen to move like Michael Jackson) you just need to move your legs, rather than your upper body (much).
Whereas to go from the bottom to the top you need to move your entire body.
Newton's First and Second Laws.
(isn't it!?
)I'm dying to be proven wrong, but I'm pretty sure Newton is with me.
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