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# moving Lift or Not

chatrack
Hi ,

Suppose you are in a closed lift. You dont know whether it is moving.

How will you understand that if the lift is:

1. Moving down uniformily
or
2. Moving Up unifirmily
or
3. Lift is at rest

The commen thing in above three cases is that
we do not feel any change in weight in our body.

Then how will you find motion status of the lift

Indi
Do you mean closed in the theoretical sense, or just closed in the common sense. Because even though you can't see out of a closed elevator when it is moving, any signal that gets in can be used to measure motion, provided you know that the signal is stationary (or, if not, you know how it is moving). A sensitive enough meter to measure the magnetic field (for instance), could tell you how the elevator is moving with respect to the Earth.

Another technique, if you can take a reading before the elevator is sealed, is to use inertial measurements to keep track of your movements. This is how rockets' and weapons' guidance systems work.

But you have a problem of definition. You ask how you will know the lift is moving... but you never say relative to what. "Moving" means nothing without a frame of reference.
nilsmo
If everything inside the lift can't interact with anything from outside the lift, then you'd have no way of detecting whether the outside is moving at 10000m/s or 0m/s, so you can't determine the "motion status."

If you can somehow interact with stuff outside the lift (e.g. a magnetic field as Indi said) then you may be able to determine how fast the lift is moving.

Implicitly I've assumed moving means moving RELATIVE to stuff outside the lift. This is the matter of definition Indi mentioned. For example, if I'm walking forward at 5 m/s, you could say I'm NOT moving but that the Earth is moving backward at 5 m/s.
ocalhoun
You just listen for the mechanical noise of the pulleys and cables moving it of course! ^.^

In addition to magnetic fields, you could also measure gravitational fields, with sensitive enough measurements.
If there is no gravitational field present, you've at least solved part of the problem: you still don't know if it is at rest or moving, but without gravity, asking if it is going up or down is meaningless.
xalophus
 chatrack wrote: Suppose you are in a closed lift. You dont know whether it is moving.

Call for help !

Because if you're in a closed lift and can't remember how or why you got there - figuring out if you're moving or stationary should be the least of your concerns.

If you're doing this as part of some sort of experiment for the sake of science - first you'll need to define your frame of reference.
I'll assume you mean the movement of the lift as relative to the rest of the building.
Next you'll need to tell us what kind of outside information(?) can reach you at the moment (air pressure ? magnetic fields ? electro-magnetic waves ?)
Based on this input you can employ various instruments to get the desired result.
chatrack
I agree with Indi about the motion detecting techequnic. I thought "Inertia" which is a property of moving body only, could be used for detection.
nilsmo
chatrack, inertia is a property of a body acted upon by a force (meaning it's accelerating). That's not the same thing as a moving body.
chatrack
Hi,
Can we use Momentum of a body in the lift for finding its present velocity?

P = M V, since mass remain constant, if we can some how measure
P is it possible to find V ?
ocalhoun
 chatrack wrote: Hi, Can we use Momentum of a body in the lift for finding its present velocity? P = M V, since mass remain constant, if we can some how measure P is it possible to find V ?

That only works with relative velocity.
While in ordinary situations on Earth, we can just use velocity relative to the ground, but if the ground is not available as a reference, there is no 'absolute' velocity.
chatrack
Hi,
if I have a handy radar (radio type) , i have a chance to get an answer.
Dennise
Note the time on your watch when you enter the lift and your entry floor.

After the lift door closes, wait a few hours and if nothing happens, you can assume your speed was zero.

If you hear/feel a crash, note if you experience a sudden temporary increase or decrease in your weight at that time. If you feel a weight increase, you have traveled down and stopped, and if you feel a weight reduction you have traveled up and stopped. In either case note the time of the crash. Note also that in either case the sudden stop results in acceleration.

If you survive the crash, and know the floor heights, you can work out your average speed up or down.

The lesson here of course is that without acceleration, and without external knowledge, one cannot sense motion at all inside the lift. Also, even with acceleration, inside the closed lift one cannot distinguish any difference whatsoever whether the lift is in in a (static) gravitational field (e.g. Earth's gravity) or if the lift is accelerating by position change. This is Einstein's equivalence principle of his general relativity theory.
Bikerman
 Dennise wrote: Note the time on your watch when you enter the lift and your entry floor. After the lift door closes, wait a few hours and if nothing happens, you can assume your speed was zero.
No you can't. You might be able to assume that the speed of the lift and the surroundings are constant - that is about all.
 Quote: If you hear/feel a crash, note if you experience a sudden temporary increase or decrease in your weight at that time. If you feel a weight increase, you have traveled down and stopped, and if you feel a weight reduction you have traveled up and stopped. In either case note the time of the crash. Note also that in either case the sudden stop results in acceleration.
In the first case you have experienced deceleration and in the second acceleration. The time will not be much use without more information, since time, like speed, is relative. If your lift can travel at 0.9c then your time will be different to my time and to the time when you stop moving...as, indeed, will be the distance travelled.
 Quote: If you survive the crash, and know the floor heights, you can work out your average speed up or down.
True.
 Quote: The lesson here of course is that without acceleration, and without external knowledge, one cannot sense motion at all inside the lift. Also, even with acceleration, inside the closed lift one cannot distinguish any difference whatsoever whether the lift is in in a (static) gravitational field (e.g. Earth's gravity) or if the lift is accelerating by position change. This is Einstein's equivalence principle of his general relativity theory.
Close enough - basically you cannot distinguish between a gravitational field and acceleration produced by an external force. The profound corollary of that is that inertia and mass cannot be distinguished either (that is far less obvious but equally important).
iman
according to relativity, you can't really tell if you're moving or not.
Bikerman
 iman wrote: according to relativity, you can't really tell if you're moving or not.
Exactly - so you cannot say you are stationary, but you can say you are in uniform (non inertial) motion (which would include being stationary).
chatrack
Hi,

If we moove up in a lift, value of g is decreasing (in small amount). So if you have
a penudulum, you can find whether you are moving up or down in a lift.

Is this a right way..?
Bikerman
 chatrack wrote: Hi, If we moove up in a lift, value of g is decreasing (in small amount). So if you have a penudulum, you can find whether you are moving up or down in a lift. Is this a right way..?

LOL...well, theoretically yes. The period of the pendulum is approximated by 2*pi*sqrt(L/g) where L is the length and g is local gravity. It is apparent, then, that a lower value of g will produce a longer period, and vica-versa.
In reality the difference would be extremely small and would be masked by the movement of the lift itself, but as a thought experiment yes, that would work.