Quizzes & Puzzles34 mins ago
Gravity
Nothing to do with Arubahz's post below but can someone please tell me how gravity works.
I'm sure that ive asked something similar in the past but I just cant wrap my head around this.
If an object is spinning, then it will throw things AWAY from it, Not TOWARDS it.
A kids roundabout will throw scruffy little urchins towards the outer edges, not towards its centre. The Earth is spinning and yet it somehow attracts us all towards its centre (gravity)??
My knowledge of physics is abysmal.
Ta!
I'm sure that ive asked something similar in the past but I just cant wrap my head around this.
If an object is spinning, then it will throw things AWAY from it, Not TOWARDS it.
A kids roundabout will throw scruffy little urchins towards the outer edges, not towards its centre. The Earth is spinning and yet it somehow attracts us all towards its centre (gravity)??
My knowledge of physics is abysmal.
Ta!
Answers
Best Answer
No best answer has yet been selected by nailit. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.
Hmmm http://
(They tried to tell me that holes move in a semiconductor too. Everyone knows it isn't the hole that moves.)
(They tried to tell me that holes move in a semiconductor too. Everyone knows it isn't the hole that moves.)
"...it looks as if a fictitious force isn't real."
Dictionaries are a bad place to turn to for scientific guidance anyway. But even if we accept that forces that are, really, just artefacts of your frame of reference, then that would arguably make gravity itself a fictitious force -- being merely an artefact of the shape of spacetime.
It's best not to go too far down this rabbit hole, at least not at this time of night...
I do intend to get back to this thread but it's been a long day. Maybe tomorrow.
Dictionaries are a bad place to turn to for scientific guidance anyway. But even if we accept that forces that are, really, just artefacts of your frame of reference, then that would arguably make gravity itself a fictitious force -- being merely an artefact of the shape of spacetime.
It's best not to go too far down this rabbit hole, at least not at this time of night...
I do intend to get back to this thread but it's been a long day. Maybe tomorrow.
Centrifugal force is often labelled fictitious but it is still a force which is basically a consequence of intertia. In an astronauts' training centrifuge for example the force is felt because the basket wants to go straight on but is getting pulled around in a circle so the force of being thrown out is continually felt by the occupant.
ToraToraTora //Centrifugal force is often labelled fictitious but it is still a force which is basically a consequence of intertia.//
No, it isn't a real force in an inertial frame of reference. The only real force involved in a centrifuge (once reaching constant rotational velocity) is the centripetal force that accelerates it into a rotating path and prevents the centrifuge itself flying apart.
The material being centrifuged is simply continuing in a straight line according to Newton's First Law of Motion.
Centrifugal force only exists within the non-inertial frame of reference of the centrifuge. This frame does not experience the centripetal force.
No, it isn't a real force in an inertial frame of reference. The only real force involved in a centrifuge (once reaching constant rotational velocity) is the centripetal force that accelerates it into a rotating path and prevents the centrifuge itself flying apart.
The material being centrifuged is simply continuing in a straight line according to Newton's First Law of Motion.
Centrifugal force only exists within the non-inertial frame of reference of the centrifuge. This frame does not experience the centripetal force.
davegosling // The moon's velocity is just sufficient to cause it to be receding from the Earth at about a couple of inches a year but, eventually, it will stabalise. //
Any body will orbit at a height such that the gravitational force is exactly that which provides the centripetal acceleration to maintain that orbit.
The Moon recedes because it gains kinetic energy from the Earth through the action of the tides continually forcing it into a higher orbit.
Given enough time Earth's rotation would slow until a day and a lunar month were the same period. Then, the Earth would always face the same side to Moon, as the Moon already does to the Earth.
However there isn't enough time for this to happen before the Sun becomes a Red Giant and consumes the inner planets.
Any body will orbit at a height such that the gravitational force is exactly that which provides the centripetal acceleration to maintain that orbit.
The Moon recedes because it gains kinetic energy from the Earth through the action of the tides continually forcing it into a higher orbit.
Given enough time Earth's rotation would slow until a day and a lunar month were the same period. Then, the Earth would always face the same side to Moon, as the Moon already does to the Earth.
However there isn't enough time for this to happen before the Sun becomes a Red Giant and consumes the inner planets.
I have this horrible feeling that to talk about that properly one needs to introduce Gauge Theory, and I don't particularly want to do that -- but it's a very interesting question.
Perhaps the simplest answer is that we don't know yet, but if we ever found a Graviton (particle that carries the force of Gravity) then yes, Gravity is a real force.
It's also interesting to note that -- at the most basic level, at least -- the "force equations" of electricity and gravity look almost exactly the same. Two charges (Q and q) exert a force on each other that goes as
electric force = (k * Q *q)/ (distance)^2
while two masses exert a force
gravity = (G * M * m)/(distance)^2
where k and G are some constants. Obviously there is way more to both forces but the parallels can be drawn even when you make things more complicated. The key difference is that charges can be both positive and negative, but masses can only ever be positive. So I suppose if we also observed (real) objects with negative masses, that would be another sign that gravity is a real force (or, put another way, that mass is just another kind of "charge").
I don't expect to ever see a "negative mass" object, but I thought I'd throw it out there.
Perhaps the simplest answer is that we don't know yet, but if we ever found a Graviton (particle that carries the force of Gravity) then yes, Gravity is a real force.
It's also interesting to note that -- at the most basic level, at least -- the "force equations" of electricity and gravity look almost exactly the same. Two charges (Q and q) exert a force on each other that goes as
electric force = (k * Q *q)/ (distance)^2
while two masses exert a force
gravity = (G * M * m)/(distance)^2
where k and G are some constants. Obviously there is way more to both forces but the parallels can be drawn even when you make things more complicated. The key difference is that charges can be both positive and negative, but masses can only ever be positive. So I suppose if we also observed (real) objects with negative masses, that would be another sign that gravity is a real force (or, put another way, that mass is just another kind of "charge").
I don't expect to ever see a "negative mass" object, but I thought I'd throw it out there.
hi Jim - and a good new year to you
any nearer your Ph D ? - it should be around three years since you sent up t' North
You are clearly going for internet geek of the year 2018 - early entry
why not approach gravity the way Newton ( said he ) did it ?
Newton said to things - everything goes on in a straight line at whatever speed unless a force acts on it
and 2) a force will cause acceleration
He sat under a tree in Woolsthorpe during the plague year and saw an apple fall - and then thought well it falls dfreom the branch - plop! and it falls from the top of the tree, and it would fall from the tallest chimney on the farm
so.... why doesnt the moon fall to earth ?
and so he thought a bit wiv da pencil in 'iz marf
and said ah! the moon isnt travelling in a straight as per my first law and so there IS a force to make it go out of the strainght line, and that is making it travel in a circle
and then Newton said - ergo babies where is my Nobel Prize?
and his servant said there isnt one etc etc
and then Newton said - OK I am calling the force that makes the moon circle the earth gravity
and boys, hey my big idea is that it is universe wide....
and about a century later Pierre-luly Laplace - hey you know what, I can conceive of something so large that it sucks everything in and not even light ( which we know is particular)
and he put it in his first edition
and then in his third edition he took it out ( the future reference to black holes that is!) probably b/c he thought it was stoopid
any nearer your Ph D ? - it should be around three years since you sent up t' North
You are clearly going for internet geek of the year 2018 - early entry
why not approach gravity the way Newton ( said he ) did it ?
Newton said to things - everything goes on in a straight line at whatever speed unless a force acts on it
and 2) a force will cause acceleration
He sat under a tree in Woolsthorpe during the plague year and saw an apple fall - and then thought well it falls dfreom the branch - plop! and it falls from the top of the tree, and it would fall from the tallest chimney on the farm
so.... why doesnt the moon fall to earth ?
and so he thought a bit wiv da pencil in 'iz marf
and said ah! the moon isnt travelling in a straight as per my first law and so there IS a force to make it go out of the strainght line, and that is making it travel in a circle
and then Newton said - ergo babies where is my Nobel Prize?
and his servant said there isnt one etc etc
and then Newton said - OK I am calling the force that makes the moon circle the earth gravity
and boys, hey my big idea is that it is universe wide....
and about a century later Pierre-luly Laplace - hey you know what, I can conceive of something so large that it sucks everything in and not even light ( which we know is particular)
and he put it in his first edition
and then in his third edition he took it out ( the future reference to black holes that is!) probably b/c he thought it was stoopid
The key is: Why are inertial mass and gravitational mass the same? It is true that the electromagnetic force and gravitation both obey the inverse square law, but the electromagnetic force accelerates a body according to its inertial mass, which is independent of its charge. Gravitational force also accelerates a body according to its inertial mass, but this is always equal to its mass of gravitational attraction. Why? If the answer involves gauge theory I'd still be keen to know it.
I wouldn't want to pretend that I have an answer to *that* question (ie, why inertial and gravitational masses are the same). It was this observation that prompted Einstein to develop General Relativity, and even then that theory doesn't actually answer the question per se.
Still, Gauge Theory seems relevant to the understanding of whether or not Gravity is a real force. The "real" forces you mentioned (along with the weak force) all have in common that they can be associated with a Gauge Theory, and in turn that implies that they all have some "exchange" particle associated with that Gauge Theory. The idea would then be that if one can write down a Gauge Theory of Gravity then it too would be a "real" force after all.
But maybe this is a circular argument. Maybe not. Either way it can be fascinating to think about, and ideas related to this rather naive argument are behind several active research topics at the moment, eg it's been noticed recently that there is a fairly direct link between certain calculations in the strong force and certain calculations in gravity (assuming the existence of a graviton).
It's tough to explain what's going on in so few words and I can only recommend that people look up, eg, Feynman's Lectures on Electromagnetism, that should provide a very readable introduction to the "simplest" of Gauge Theories.
Still, Gauge Theory seems relevant to the understanding of whether or not Gravity is a real force. The "real" forces you mentioned (along with the weak force) all have in common that they can be associated with a Gauge Theory, and in turn that implies that they all have some "exchange" particle associated with that Gauge Theory. The idea would then be that if one can write down a Gauge Theory of Gravity then it too would be a "real" force after all.
But maybe this is a circular argument. Maybe not. Either way it can be fascinating to think about, and ideas related to this rather naive argument are behind several active research topics at the moment, eg it's been noticed recently that there is a fairly direct link between certain calculations in the strong force and certain calculations in gravity (assuming the existence of a graviton).
It's tough to explain what's going on in so few words and I can only recommend that people look up, eg, Feynman's Lectures on Electromagnetism, that should provide a very readable introduction to the "simplest" of Gauge Theories.
Part 1
I was fortunate enough to have some very good teachers who were able to remove some of the confusion in this area. Let’s see if I can do justice to their efforts to drive something into this brain of mine…
As I see it there are three issues here
1. The confusion over centripetal/centrifugal forces and how they act
2. What is gravity (classical explanations)
3. Why is gravity (relativistic explanations)
Before getting on to any of that, can we talk about frames of reference.
From the surface of the earth, we perceive ourselves as stationary and observe the sun rise and set each day.
From outer space, we see the earth spinning on its axis and orbiting the sun.
Moving further away, we see the sun orbiting the galactic centre and the local galaxies orbiting one another, and so on.
When we try to describe a physical phenomenon, we have to be clear about the frame of reference (FoR) and whether that FoR is moving or accelerating with respect to any other aspect of the phenomenon.
For an observer on the surface of the earth, her FoR is accelerating with respect to the centre of the earth (it is moving around the centre, and therefore has a centripetal acceleration at the same time as a linear translation).
Put it another way: when we ride on a fairground Chair-o-Plane, our chair is supported on chains. When ride starts, our chair starts to fly outward “under centrifugal force”.
Let’s break that down. The person in the chair experiences nothing much unusual. She sits in the chair under her own weight and that force (her mass times the acceleration due to gravity) is reacted through the chair and the chains up to the top of the tower, and then through the structure of the ride down to earth.
As the ride starts to turn, there is very little change. She feels a small increase in the forces reacted through the chair, but that aside, enjoys a gentle ride, despite the observers on the ground seeing her flying around at alarming angles.
A physicist observing from the ground explains this by saying that the chair requires a force toward the centre, in order to maintain the circular motion. The physicist calls that a centripetal force.
We have all seen those force/vector triangles that show the mass acting as a vertical downward force; the chains acting as a restoring force at that slightly alarming angle, and another force acting horizontally.
That force is variously described as a centripetal (toward the centre) force; a centrifugal (away from the centre) force or, as I was taught in my engineering lectures, a d’Alembert force.
The physicist is correct to call it a centripetal force. A mass moving in a circular path is constantly changing direction. Velocity is a vector (needs a direction as well as magnitude), A continuously changing velocity requires the continuous application of a force. In this case, the force is always directed toward the centre. That’s as seen from the physicist’s frame of reference, watching the chairs fly around.
However, engineers prefer to analyse static situations (which are much easier than dealing with something constantly moving and changing velocity).
That means changing the frame of reference to the person in the chair.
We invoke a so-called d’Alembert force that acts away from the centre. We do this because our new frame of reference is accelerating due to the motion of the ride.
In colloquial language, that’s a centrifugal force. Yes, it’s a real force. But you have to be clear about the frame of reference before characterising it as centripetal or centrifugal.
Part 2 below....
I was fortunate enough to have some very good teachers who were able to remove some of the confusion in this area. Let’s see if I can do justice to their efforts to drive something into this brain of mine…
As I see it there are three issues here
1. The confusion over centripetal/centrifugal forces and how they act
2. What is gravity (classical explanations)
3. Why is gravity (relativistic explanations)
Before getting on to any of that, can we talk about frames of reference.
From the surface of the earth, we perceive ourselves as stationary and observe the sun rise and set each day.
From outer space, we see the earth spinning on its axis and orbiting the sun.
Moving further away, we see the sun orbiting the galactic centre and the local galaxies orbiting one another, and so on.
When we try to describe a physical phenomenon, we have to be clear about the frame of reference (FoR) and whether that FoR is moving or accelerating with respect to any other aspect of the phenomenon.
For an observer on the surface of the earth, her FoR is accelerating with respect to the centre of the earth (it is moving around the centre, and therefore has a centripetal acceleration at the same time as a linear translation).
Put it another way: when we ride on a fairground Chair-o-Plane, our chair is supported on chains. When ride starts, our chair starts to fly outward “under centrifugal force”.
Let’s break that down. The person in the chair experiences nothing much unusual. She sits in the chair under her own weight and that force (her mass times the acceleration due to gravity) is reacted through the chair and the chains up to the top of the tower, and then through the structure of the ride down to earth.
As the ride starts to turn, there is very little change. She feels a small increase in the forces reacted through the chair, but that aside, enjoys a gentle ride, despite the observers on the ground seeing her flying around at alarming angles.
A physicist observing from the ground explains this by saying that the chair requires a force toward the centre, in order to maintain the circular motion. The physicist calls that a centripetal force.
We have all seen those force/vector triangles that show the mass acting as a vertical downward force; the chains acting as a restoring force at that slightly alarming angle, and another force acting horizontally.
That force is variously described as a centripetal (toward the centre) force; a centrifugal (away from the centre) force or, as I was taught in my engineering lectures, a d’Alembert force.
The physicist is correct to call it a centripetal force. A mass moving in a circular path is constantly changing direction. Velocity is a vector (needs a direction as well as magnitude), A continuously changing velocity requires the continuous application of a force. In this case, the force is always directed toward the centre. That’s as seen from the physicist’s frame of reference, watching the chairs fly around.
However, engineers prefer to analyse static situations (which are much easier than dealing with something constantly moving and changing velocity).
That means changing the frame of reference to the person in the chair.
We invoke a so-called d’Alembert force that acts away from the centre. We do this because our new frame of reference is accelerating due to the motion of the ride.
In colloquial language, that’s a centrifugal force. Yes, it’s a real force. But you have to be clear about the frame of reference before characterising it as centripetal or centrifugal.
Part 2 below....
Part 2
2. Apologies for this, but the classical explanation of gravity is given in the equation F = G(M1 * M2)/r^2.
That is to say, the force between two (massive) objects M1 and M2 is proportional to their combined weights multiplied together and inversely proportional to the square of the distance between them. The constant of proportionality is given the name ‘big G’, the gravitational constant.
OK, enough maths…
Until about 1915, this was a kind of magic formula. We knew that gravity happens, and we could estimate the size of the gravitational force, but we didn’t know why it happens.
How is it that a large lump of mass (such as the earth) can exert a force on something (such as the moon) that lies a quarter of a million miles away across a vacuum?
This was one of those questions that physicists had put to one side as being a bit too difficult, until a chap called Einstein came along.
Before we get on to that, here is the answer to nailit’s original question.
Yes, the earth is spinning about its axis. Just as in the Chair-o-Plane example, if we are to keep up with it, then we need something to stop us flying off the surface. In the Chair-o-plane, it was those chains.
Think about someone standing at the North Pole and another person standing on the equator.
At the Pole, there is no spinning force tending to push our imaginary person out into Space. To use the kids’ roundabout example, it is like standing right at the middle of the roundabout.
At the equator, the force needed to keep the person from flying out into space is mass times 0.02. The gravitational force keeping our feet on the ground is mass times 9.98. The attractive force due to gravity is about 500 times stronger than needed to stop us flying off the surface.
3. The final aspect of this is, Why does gravity exist.
As I said, until Einstein published his paper on general relativity in 1915, that was one of the questions we simply did not how to answer.
He changed the thinking away from classical mechanics to something very different in which space and time are wrapped up together and can be distorted by large amounts of energy or large amounts of mass.
It’s often explained by the rubber sheet analogy in which we place a heavy ball on a large, horizontal elastic sheet. That weight distorts the sheet, so that when we roll a smaller ball across the sheet, the distortion due to the big ball affects the path of the smaller ball.
The 2-dimensional elastic sheet is distorted into a third dimension (up/down). However, space-time is already at least four dimensions, so the distortion adds at least one more dimension, meaning that we have to adopt quite advanced maths to describe these phenomena, and that, I respectfully suggest, it beyond both my ability to explain, and most readers’ boredom threshholds.
Hope it helps - sorry about the length
2. Apologies for this, but the classical explanation of gravity is given in the equation F = G(M1 * M2)/r^2.
That is to say, the force between two (massive) objects M1 and M2 is proportional to their combined weights multiplied together and inversely proportional to the square of the distance between them. The constant of proportionality is given the name ‘big G’, the gravitational constant.
OK, enough maths…
Until about 1915, this was a kind of magic formula. We knew that gravity happens, and we could estimate the size of the gravitational force, but we didn’t know why it happens.
How is it that a large lump of mass (such as the earth) can exert a force on something (such as the moon) that lies a quarter of a million miles away across a vacuum?
This was one of those questions that physicists had put to one side as being a bit too difficult, until a chap called Einstein came along.
Before we get on to that, here is the answer to nailit’s original question.
Yes, the earth is spinning about its axis. Just as in the Chair-o-Plane example, if we are to keep up with it, then we need something to stop us flying off the surface. In the Chair-o-plane, it was those chains.
Think about someone standing at the North Pole and another person standing on the equator.
At the Pole, there is no spinning force tending to push our imaginary person out into Space. To use the kids’ roundabout example, it is like standing right at the middle of the roundabout.
At the equator, the force needed to keep the person from flying out into space is mass times 0.02. The gravitational force keeping our feet on the ground is mass times 9.98. The attractive force due to gravity is about 500 times stronger than needed to stop us flying off the surface.
3. The final aspect of this is, Why does gravity exist.
As I said, until Einstein published his paper on general relativity in 1915, that was one of the questions we simply did not how to answer.
He changed the thinking away from classical mechanics to something very different in which space and time are wrapped up together and can be distorted by large amounts of energy or large amounts of mass.
It’s often explained by the rubber sheet analogy in which we place a heavy ball on a large, horizontal elastic sheet. That weight distorts the sheet, so that when we roll a smaller ball across the sheet, the distortion due to the big ball affects the path of the smaller ball.
The 2-dimensional elastic sheet is distorted into a third dimension (up/down). However, space-time is already at least four dimensions, so the distortion adds at least one more dimension, meaning that we have to adopt quite advanced maths to describe these phenomena, and that, I respectfully suggest, it beyond both my ability to explain, and most readers’ boredom threshholds.
Hope it helps - sorry about the length
