Centrifugal forces ' appear ' to act opposite to gravity . How is this possible?

[studiot]

 

imatfaal

If centripetal (inwards radial) was balanced (ie by an outwards radial) then the object would continue in a straight line! The Centripetal force is calculated as that force needed to accelerate the mass inwards - ie to stop it continuing in a straight line.

 

Well put, I keep telling Mike that, and it was a point I made several times in the now closed thread.

 

+1

 

My emphasis added.

Edited April 2, 2015 by studiot

[swansont]

?

 

Here is a Wikipedia reference to it. but describing the Centrifugal force. .

 

http://www.3rd1000.com/chem101/chem104i.htm

 

Note the mention of his view of centrifugal force in this context .

 

Quote "

 

The Bohr model of an atom postulated a structure in which a single electron moved in a circular orbit around the central nucleus, much as a satellite orbits the earth or the planets orbit the sun. Satellites may circle the earth at any distance outside the atmosphere; their orbits may have any radius. As a satellite travels in its orbit there is a balance between the inward force of gravitation, mm'g/r2, and the outward or centrifugal force of the moving satellite, mv2/r. In these equations m is the mass of the satellite, m' is the mass of the central earth, r is the radius of the orbit, and v is the velocity of the satellite in its orbit. The forces are equal in a stable orbit; mm'g/r2 = mv2/r gives by rearrangement v2r = m'g. Since the mass of the earth m'and the gravitational constant g are both constants, a satellite can assume an orbital radius if and only if it possesses the appropriate velocity. Acceleration of the satellite must therefore increase both its velocity and its orbital radius so it moves outward, while deceleration will move it inwards. Astronauts use this procedure, decelerating their capsules or shuttles by firing rockets to move inward until they reenter atmosphere.

 

" Unquote

 

[in the atomic orbit version , the inward force centripetal is electrostatic force between electron and nucleus , not gravity]

 

Mike

 

Two possibilities: You've either discovered someone else who doesn't understand circular motion (congratulations, this is an oft-misunderstood subject, and this is a chemist, not a physicist), or they are analyzing it from the object's frame, which requires a pseuoforce force in order to "balance", because in the electrons's frame, it's in linear motion.

 

You can do one or the other, but you can't mix the two. I personally dislike the latter method, because invariably people misapply it. It only works for a object moving in a circular path..

[Strange]

Strange was also indicating that " electromagnetic " might be ( All there is )

 

That is not what I said (or not what I intended to say, anyway).

[Mike Smith Cosmos]

Two possibilities: You've either discovered someone else who doesn't understand circular motion (congratulations, this is an oft-misunderstood subject, and this is a chemist, not a physicist), or they are analyzing it from the object's frame, which requires a pseuoforce force in order to "balance", because in the electrons's frame, it's in linear motion.

 

You can do one or the other, but you can't mix the two. I personally dislike the latter method, because invariably people misapply it. It only works for a object moving in a circular path..

Yes , but the early scientists , 1700,s 1800,s and early 1900's did go for using more standard classical means of motion of particles before we all got tied up in quantum theory and relativity. I appreciate one has to move on to understand quantum and relativistic effects.

 

Because circular motion is difficult , to conduct experiments with as ( a ) it's moving in a complete cycle , so effects can cancel . And it's extremely difficult to make measurements on a small scale ( unless you are traveling round with the instruments, like on an astronauts test centrifuge ,( b) or for that matter measure over small or discrete , small, sections of travel . (C ) we do not naturally go round in circles , whereas we do move strait forward. , in strait lines ( unless we have had a drink ,that is ) .

 

------------------------

 

I am not sure ,I understand fully what your second point is . Namely " or they are analyzing it from the object's frame, which requires a pseuoforce force in order to "balance", because in the electrons's frame, it's in linear motion.

 

You can do one or the other, but you can't mix the two. I personally dislike the latter method, because invariably people misapply it. It only works for a object moving in a circular path.." .??

 

-------------------------

 

Mike

Edited April 3, 2015 by Mike Smith Cosmos

[studiot]

 

because in the electrons's frame, it's in linear motion.

 

 

Suppose there were two observers travelling in the dark so they could not observe anything external to themselves at all.

 

Observer A is travelling along a wall (he can't see) that emits parallel jets of air always pushing him sideways to his left.

 

Observer B is subject to an electrostatic attraction towards her left so that she travels in a circle.

 

How would each observer determine what sort of motion they were executing, given that they can't see anything of the outside universe.?

Edited April 4, 2015 by studiot

[swansont]

You can look at an even simpler example. You're on a merry-go-round/carousel/lazy susan kind of device, but you can't directly measure it. You feel, much like we do on earth, like you aren't moving at all. If you throw a ball to someone else, you notice the ball deflects. That's an indication that you are in an accelerated frame. But you want to pretend you aren't, so you can use Newton's laws of motion, so you make up a force to explain the curved motion of a tossed ball.

 

 

If viewed from an inertial frame, you can see that the ball does not curve.

 

Similarly, if you are looking a your motion around the circle, you feel something pushing in on you, and you push back on it. If you want to pretend you are in an inertial frame and traveling in a straight line, then you have to make up a centrifugal force to balance the centripetal force. but when viewed from an inertial frame, there is only one force — the one pushing you toward the center.

 

The problem arises when people try and say there is a centrifugal force in an inertial frame. There isn't. Zero force means straight line motion. You can;t have zero force and circular motion. That violates Newton's laws. It's wrong.

[Mike Smith Cosmos]

You can look at an even simpler example. You're on a merry-go-round/carousel/lazy susan kind of device, but you can't directly measure it. You feel, much like we do on earth, like you aren't moving at all. If you throw a ball to someone else, you notice the ball deflects. That's an indication that you are in an accelerated frame. But you want to pretend you aren't, so you can use Newton's laws of motion, so you make up a force to explain the curved motion of a tossed ball.

 

 

If viewed from an inertial frame, you can see that the ball does not curve.

 

Similarly, if you are looking a your motion around the circle, you feel something pushing in on you, and you push back on it. If you want to pretend you are in an inertial frame and traveling in a straight line, then you have to make up a centrifugal force to balance the centripetal force. but when viewed from an inertial frame, there is only one force the one pushing you toward the center.

 

The problem arises when people try and say there is a centrifugal force in an inertial frame. There isn't. Zero force means straight line motion. You can;t have zero force and circular motion. That violates Newton's laws. It's wrong.

Which frame am I in , down here on the surface of the earth ? And am I in the same frame if I am hovering just a few feet above the earth ? And what frame am I in , if I vibrate sideways back and forth ? Or travel round in a bucket as water , being thrown around by someone earth bound , a ) when swung around parallel to the earth and b) when swung up into the air and around down towards the earth and up again , and so on?

 

Mike

Edited April 6, 2015 by Mike Smith Cosmos

[swansont]

Which frame am I in , down here on the surface of the earth ? And am I in the same frame if I am hovering just a few feet above the earth ?

 

You are in the earth's frame.

 

 

And what frame am I in , if I vibrate sideways back and forth ? Or travel round in a bucket as water , being thrown around by someone earth bound , a ) when swung around parallel to the earth and b) when swung up into the air and around down towards the earth and up again , and so on?

You are in whichever frame you have described.

 

Newton's first law tells us that an object at rest remains at rest, and an object in uniform motion remains in uniform motion, unless an external force is acting upon it.

 

If that is not happening, i.e. things spontaneously speed up or slow down, or move in curved paths, with no discernible force on them, then you are not in an inertial frame, and Newton's laws do not apply.

[Mike Smith Cosmos]

You are in the earth's frame.

 

 

You are in whichever frame you have described.

Newton's first law tells us that an object at rest remains at rest, and an object in uniform motion remains in uniform motion, unless an external force is acting upon it.

If that is not happening, i.e. things spontaneously speed up or slow down, or move in curved paths, with no discernible force on them, then you are not in an inertial frame, and Newton's laws do not apply.

So when I find myself NOT in an inertial frame , whose laws if NOT Newtons laws must I use ?

[ajb]

So when I find myself NOT in an inertial frame , whose laws if NOT Newtons laws must I use ?

You can 'modify' Newton's laws so that they do hold by introducing fictitious forces. That is the whole point of these forces, they are due to 'poor' choice of coordinates.

[swansont]

So when I find myself NOT in an inertial frame , whose laws if NOT Newtons laws must I use ?

 

You can 'modify' Newton's laws so that they do hold by introducing fictitious forces. That is the whole point of these forces, they are due to 'poor' choice of coordinates.

 

 

Another option is to just use an inertial frame. The nice thing is that these laws work in any inertial frame, so you can pick the most convenient one (e.g. one object at rest, or center of mass, or whatever).

[Mike Smith Cosmos]

Another option is to just use an inertial frame. The nice thing is that these laws work in any inertial frame, so you can pick the most convenient one (e.g. one object at rest, or center of mass, or whatever).

I am a little unsure , which , or how I pick or apply the ' chosen ' inertial frame for my investigation of what really is happening , during a partial arc say 8 inches ( 4" in either direction ) 3 meters above my head , travelling at different speeds , say 17,000 mph , 22,000 mph , and zero mph. In either and both at different times , of opposite directions . Yet keeping it for all cases in inertial frame/s . So that I may use Newtons laws. Or does the Arc / part circular , exclude me using inertial frames ?

 

Mike

Edited April 6, 2015 by Mike Smith Cosmos

[swansont]

I am a little unsure , which , or how I pick or apply the ' chosen ' inertial frame for my investigation of what really is happening , during a partial arc say 8 inches ( 4" in either direction ) 3 meters above my head , travelling at different speeds , say 17,000 mph , 22,000 mph , and zero mph. In either and both at different times , of opposite directions . Yet keeping it for all cases in inertial frame/s . So that I may use Newtons laws. Or does the Arc / part circular , exclude me using inertial frames ?

 

Mike

 

If it changes direction or speed it's not in an inertial frame. You have to analyze it from a frame that does not do these things (or where any such effects are so small that they can be neglected)

[Mike Smith Cosmos]

If it changes direction or speed it's not in an inertial frame. You have to analyze it from a frame that does not do these things (or where any such effects are so small that they can be neglected)

The last time I flew (aeroplane as passenger ) . When it was time for take off. I carried a heavy object in my hand. Like a concrete lemon . ( one used in an attempt , to drop from the top of the leaning tower of Pisa . ) .

 

At the end of the runway , the engines revved up. It set off down the runway. I tossed the lemon up , in my seat ( difficult ) . It went strait up and strait down . Most of the way down the early part of the runway . Strait up and strait down , then as it got a good way down the runway , the lemon would go up then down at an angle into my chest , which I assumed was the acceleration bit as it took off, . I had never done this experiment on the plane before. During the times without the lemon , the runway to takeoff and into the air , 'felt ' the same all the way into the air except it felt smoother in the air.

 

I presumed my experiment proved that it must have been constant velocity going down runway ( up and down lemon) and accelerating when ( into chest with lemon) . Then there was a force ( F = ma ) .

 

So I suppose going down the runway was in an inertial frame , and late runway and into air a different frame . Of course there was also a change in direction. Going from horizontal ( runway) to climb at 20 degrees . Although I first noticed the lemon falling into my chest before the end of the runway .

 

Is that right , then to do with frames ?

 

Mike

Edited April 6, 2015 by Mike Smith Cosmos

[swansont]

The last time I flew (aeroplane as passenger ) . When it was time for take off. I carried a heavy object in my hand. Like a concrete lemon . ( one used in an attempt , to drop from the top of the leaning tower of Pisa . ) .

 

At the end of the runway , the engines revved up. It set off down the runway. I tossed the lemon up , in my seat ( difficult ) . It went strait up and strait down . Most of the way down the early part of the runway . Strait up and strait down , then as it got a good way down the runway , the lemon would go up then down at an angle into my chest , which I assumed was the acceleration bit as it took off, . I had never done this experiment on the plane before. During the times without the lemon , the runway to takeoff and into the air , 'felt ' the same all the way into the air except it felt smoother in the air.

 

I presumed my experiment proved that it must have been constant velocity going down runway ( up and down lemon) and accelerating when ( into chest with lemon) . Then there was a force ( F = ma ) .

 

So I suppose going down the runway was in an inertial frame , and late runway and into air a different frame . Of course there was also a change in direction. Going from horizontal ( runway) to climb at 20 degrees . Although I first noticed the lemon falling into my chest before the end of the runway .

 

Is that right , then to do with frames ?

 

Mike

 

You were accelerating at least some of the time as you moved down the runway — speeding up — but it may be that this was too gentle to notice in such an experiment. You were approximately inertial.

 

So when the lemon went up and landed on your chest, there was no actual force on it, even though to you it appeared to deflect. Because you were accelerating. In your frame it looked like there was a fictitious force acting on the lemon.

[MigL]

This is the example I always use to describe fictitious forces in accelerated frames Mike...

 

Take an old fashioned record player and put a round piece of paper on it. Spin it up so it is rotating clockwise, then get a pen and draw a radial line from the center spindle to the circumference. In your inertial frame ( not spinning clockwise ) you draw a straight line directly from center to edge, but in the rotating frame of the sheet of paper, what's actually drawn by the pen is a counterclockwise spiral. As if there was a force pushing the pen in that direction.

If you were a microscopic inhabitant of this flat spinning world, you would see the pen draw out this counterclockwise spiral and no-one could convince you that there wasn't a force pushing ( or pulling ) it in that direction. This we know as a fictitious force that is solely due to the spinning ( accelerated ) frame of reference.

Since the Earth is spinning, the fictitious force due to its rotation is called Coriolis force (after Gaspard Coriolis ) and wiki has this very same explanation.

If your frame ( not inertial ) is inside a car making a high speed left turn, that is your rotating frame of reference, and the fictitious force you 'feel', seems to push you to the right, and is called centrifugal force. From an inertial frame outside the car ( person standing on the sidewalk ) there is no force pushing you to the right, rather it is the car door constraining you to accelerate to the left. And this real force is called centripetal.

( no, there were no French scientists or mathematicians called Centrifuge or Centripete )

[tar]

Thread,

 

So if a ton of stuff wheighs 1998 pounds on the equator and 2000 pounds on the North Pole, is it not a real force lifting the ton off the scale a little, on the equator, due to the spin of the Earthly carosel?

 

Regards, TAR

[Spyman]

swansont has explained this already in post #5 and #7 on the first page.

[DrP]

Tar - no, it weighs less due to there being less force acting on it by gravity as it is further from the centre of the earth.

[MigL]

That's a non-answer DrP.

Why is it further from the centre of the Earth ?

Does dirt and rock weigh less at the equator, and so 'sits higher', enabling the Earth's radius to be greater at the equator than the poles ?

 

I prefer Spyman's reference to Swansont's answers.

[tar]

Spyman,

 

I sort of get Swansont's explanation in #5 and #7, but the choice of coordinates to look at the thing from an inertial frame, seems somewhat ignorant of the reality of the motions and accelerations that cause the "force" in the first place.

 

The two components that confuse me the most about this are what is a straight line, and what is an outside force.

 

In imagining what is happening to a spot on the Earth, and what forces are being applied to it, what are outside forces and what are fictitious forces, it is difficult to ignore the "reasons" and the mechanisms that cause a thing to move this way or that. There are certain realities that cause one to be ejected from the merry-go-round. That "force" one to the outside. These same reasons, albeit because you are in an accelerated frame, are evident and present when you are a ton of stuff on the equator, as opposed to a ton of stuff on the pole. As a ton of stuff on the pole, you are clinging tightly to the center of the roundabout, where it is "easier" to oppose the outward pull. On the equator, you are hanging off the outside of the thing.

 

Regards, TAR

[Spyman]

The reason and mechanism you are looking for is Inertia:

Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is the tendency of objects to keep moving in a straight line at constant velocity. The principle of inertia is one of the fundamental principles of classical physics that are used to describe the motion of objects and how they are affected by applied forces. Inertia comes from the Latin word, iners, meaning idle, sluggish. Inertia is one of the primary manifestations of mass, which is a quantitative property of physical systems. Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:

 

The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

http://en.wikipedia.org/wiki/Inertia

 

When the merry-go-round speeds up you are accelerated with it and gain momentum, there is no force pushing you outward, it's your own inertia that wants to continue in a straight line.

 

It's the same with any object rotating with Earth on the surface of the equator, it has momentum which wants to continue straight forward and a part of gravity is used up countering it.

[swansont]

Spyman,

 

I sort of get Swansont's explanation in #5 and #7, but the choice of coordinates to look at the thing from an inertial frame, seems somewhat ignorant of the reality of the motions and accelerations that cause the "force" in the first place.

 

The two components that confuse me the most about this are what is a straight line, and what is an outside force.

 

In imagining what is happening to a spot on the Earth, and what forces are being applied to it, what are outside forces and what are fictitious forces, it is difficult to ignore the "reasons" and the mechanisms that cause a thing to move this way or that. There are certain realities that cause one to be ejected from the merry-go-round. That "force" one to the outside. These same reasons, albeit because you are in an accelerated frame, are evident and present when you are a ton of stuff on the equator, as opposed to a ton of stuff on the pole. As a ton of stuff on the pole, you are clinging tightly to the center of the roundabout, where it is "easier" to oppose the outward pull. On the equator, you are hanging off the outside of the thing.

 

Regards, TAR

 

 

Newton's first law tells you when you can invoke Newtonian force analysis: no force means either you are at rest, or moving in a straight line. And for classical physics problems your choice of forces are limited to gravitational and electromagnetic.

 

If you are in another situation, you can't apply Newton's laws. They don't work. There is no force that ejects you radially from a merry-go-round. As soon as you leave it you travel on a tangent to the circle. Not outward.

[tar]

SwansonT,

 

Just tried to draw a straight line that would take me tangent to a point. It looked exactly like radially outward.

 

When you are at the center of the merry-go-round, the only way to go is radially outward. Or at the North Pole. If you draw a straight line tangent to a point, it IS a line going radially outward.

 

Regards, TAR

[swansont]

SwansonT,

 

Just tried to draw a straight line that would take me tangent to a point. It looked exactly like radially outward.

When you leave the merry-go-round, you leave at a tangent. Not radially.

 

When you are at the center of the merry-go-round, the only way to go is radially outward. Or at the North Pole. If you draw a straight line tangent to a point, it IS a line going radially outward.

 

Regards, TAR

If you traced your path from the POV of an inertial frame, it would be a spiral. If you did it with no centripetal force at all (i.e. you removed radial friction or confinement), it would be a straight line.

 

Using "radial" and "tangent" the way you did makes my head hurt. You can't have a tangent to a point, you have a tangent to a curve, and it will be perpendicular to the line that is drawn radially outward.

 

If you think that there is a force outward, how does a trebuchet or catapult work?