Gravity question
Discussion
Eric Mc said:
Is an object falling around the earth (i.e. in orbit) undergoing the same accelerative force as an object falling directly towards the ground?
They experience a downwards accelerative force in addition to their sideways direction of motion.Acceleration is rate of change of velocity; velocity is a vector quantity so it has direction and magnitude. Any object that maintains a constant speed (a scalar quantity having only magnitude) but changes direction, undergoes acceleration. As an object in orbit does not travel in a straight line, it must be accelerating due to a net force acting upon it.
Eric Mc said:
Would a person in a spacecraft falling around the earth float in the spacecraft (i.e. experience Zero G)
Judging by the NASA video clips of people on shuttles and the ISS, yes.Eric Mc said:
Would a person in a spacecraft falling directly towards the ground also experience Zero G?
Judging by the method of training astronauts by sticking them in a big jet and pointing it nose down, yes (they experience free fall).That's what I thought 
However, I can now see that the acceleration being experienced by the falling object - whether it's a spacecraft in orbit or a lift falling directly towards the earth will be different. The occupants won't notice any difference as they are falling inside the falling object - so will just float about until they hit something.
However, I can now see that the acceleration being experienced by the falling object - whether it's a spacecraft in orbit or a lift falling directly towards the earth will be different. The occupants won't notice any difference as they are falling inside the falling object - so will just float about until they hit something.xRIEx said:
Judging by the method of training astronauts by sticking them in a big jet and pointing it nose down, yes (they experience free fall).
To be precise, the vomit comet simulates a weightless environment by climbing steeply and then performing a 1G negative pushover. If the simply pointed at earth, stick centred, then passengers would experience an effect similar to sitting on a sloped floor. If they pointed straight down passengers would experience greater than 1G as the aircraft's accelerative force would be gravity plus engine thrust, with no opposing lift component. EDIT: No that's wrong. The aircraft would experience greater than 1G, unsecured passengers would experience 1G (plus a bit because air in the fuselage would push on them) until they collided with the rear bulkhead.Eric, my arguments counter to yours aren't centred on the absolute truth of items orbiting a planet's centre (whether in space or ground level) but are instead focussed on the practical value of a ground based G meter, which IMO would take a fixed position on the Earth's surface as its point of reference. So while you and that reference point may be constantly accelerating around the earth's centre, to be of practical use a G-meter would, I think want to show you how you are moving compared to some point of situational relevance. E.G. towards or away from the ground, or centripetal acceleration towards the centre of a bend on a race track. In other words, I'm equating equilibrium to a stable orbit.
Edited by kiseca on Monday 17th August 10:33
Edited by kiseca on Monday 17th August 10:35
I see what you mean. In project Gemini back in the 1960s they tried rotating the Gemini capsule when tethered to an unmanned Agena around the common centre of gravity and they found the situation to be quite unstable due to the gravity gradiant i.e. both objects were falling at slightly different rates because they were in (very) slightly different orbits.
Moonhawk said:
Eric Mc said:
What would it read if you are falling around the earth in earth orbit? It's EXACTLY the same as falling on earth.
Something in orbit is not accelerating - it is falling at constant speed - therefore it will read zero.xRIEx said:
Acceleration is rate of change of velocity; velocity is a vector quantity so it has direction and magnitude. Any object that maintains a constant speed (a scalar quantity having only magnitude) but changes direction, undergoes acceleration.
Quite so.Nimby said:
Ali G said:
Nimby said:
Would a neutrally-buoyant diver in a tank of water be able to tell if the tank was falling or spinning in a centrifuge?
It depends...
Nice example of acceleration and frame of reference from 0:54 - https://www.youtube.com/watch?v=9-eEq1uhCVE
Secondly: Re diver in centrifuge...
https://en.wikipedia.org/wiki/Laboratory_centrifug...
Diver was in 'equilibrium' due to equalisation of densities. Densities of different substances may be affected 'differently' under changes of pressure (and temperature).
Given appropriate centrifuge revs - diver may no longer be in 'equilibrium' and may be then be aware of circumstances through physical sensation by contact with extremities of tank.
Diver may however be unconcsious well before then, so be unable to tell!
https://en.wikipedia.org/wiki/Laboratory_centrifug...
Diver was in 'equilibrium' due to equalisation of densities. Densities of different substances may be affected 'differently' under changes of pressure (and temperature).
Given appropriate centrifuge revs - diver may no longer be in 'equilibrium' and may be then be aware of circumstances through physical sensation by contact with extremities of tank.
Diver may however be unconcsious well before then, so be unable to tell!
Ali G said:
Secondly: Re diver in centrifuge...
https://en.wikipedia.org/wiki/Laboratory_centrifug...
Diver was in 'equilibrium' due to equalisation of densities. Densities of different substances may be affected 'differently' under changes of pressure (and temperature).
Given appropriate centrifuge revs - diver may no longer be in 'equilibrium' and may be then be aware of circumstances through physical sensation by contact with extremities of tank.
Diver may however be unconcsious well before then, so be unable to tell!
Small diver.https://en.wikipedia.org/wiki/Laboratory_centrifug...
Diver was in 'equilibrium' due to equalisation of densities. Densities of different substances may be affected 'differently' under changes of pressure (and temperature).
Given appropriate centrifuge revs - diver may no longer be in 'equilibrium' and may be then be aware of circumstances through physical sensation by contact with extremities of tank.
Diver may however be unconcsious well before then, so be unable to tell!
A G meter in an elevator - http://www.pistonheads.com/gassing/topic.asp?h=0&a... - at 160 km would still read a +ve value whereas one flying past on the ISS would read zero. The elevator would have to be 22,000 miles high tfor the meter o read 0 G. And then how would one get down?
Vipers said:
Small diver.
I was thinking more this. It's actually a question I had in one of my first job interviews, and I've used on others since. (Actually it was a goldfish in a bowl on a rollercoaster). One of those where it's more to do with how you/the group approach the problem rather than actual knowledge of physics and biology.
AFAIK the diver would "feel" the acceleration in both cases as the labyrinth organs in the inner ear would still work normally. In the centrifuge there would be differential forces on tissues of different density, and a Coriolis effect depending on the radius.
I wonder how many g something pretty homogeneous like a jellyfish could withstand in a centrifuge?
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