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Some comments on centrifugal and centripetal forces - Page 2

post #31 of 53
Here's another way of thinking about centrifugal force, along the lines of what was posted earlier...

If the car carrying the slick-britches passenger hits a 10-foot high concrete embankment head-on, rather than encounter the centripetal forces of turning tires on the pavement, which direction does the passenger go? Yep, back of seat, baby. Same principle, just different "shape" of the force acting on the mass in motion. If the force encountered by the forward inertia of a mass is perpendicular to the direction of travel - BAM as Emeril would say. This is where it gets a little more complex than just looking at CoM in a uni-dimensional paradigm.

Now think about a skier sliding along a curved wall 3 feet high...
Forward motion of the upper body mass is going to continually carry that portion of the skier's body to the outside of the arc traveled by the lower body. The lower parts of the body will be accelerated around the curve. As long as the skier is strong enough in the mid section, he/she can stand up straight for the entire ride. Otherwise, Gumby time. Or, maybe even a roll-over with the skier flying over the top of the wall.

Now, if that curved wall is 8 feet high, almost everyone can slide right around the curved path standing straight up. (Assuming we still have on our "slick suit"... i.e. discounting friction issues.) The centripetal forces are aligned with all points of the mass (skier).

In the GS turns discussed in the other thread, the centripetal "wall height" is only the point of contact between the snow and the ski edge- a few precious mm's high. There is no wall to slide around. Therefore, Ghost and Si are right about stacking against the centripetal forces by using the ski, bindings, boots, and skeleton to "align" more points of the mass in a plane consistent with the centripetal force. (i.e. inclination and angulation) This alignment doesn't happen because of centripetal or centrifugal forces, we choose to make it happen by how we move our body. However, physics dictates how we must move our body or end up like a ball on the end of a string that has been cut.
post #32 of 53
Physicsman,
Bringing up that infamous Daslider thread reminds me: did you ever try out that ski we talked about, with which it is possible to carve turns - even though it has no sidecut but only permanent reverse camber? Of course I am referring to a grass ski.
However it doesn't sound like you have much spare time at present...

Here's a couple of questions for general discussion - out of the two aforementioned forces, can one exist without the other? And when starting a turn, which one occurs first?
post #33 of 53
Wasn't sure if you only wanted response from Physicsman, but I'll bite...

Quote:
Here's a couple of questions for general discussion - out of the two aforementioned forces, can one exist without the other?
As physics currently defines them, I don't think so.

Quote:
And when starting a turn, which one occurs first?
Ah, the old "chicken and the egg" debate. Or maybe more appropriate... the debate over zero and infinity. Solve this one and I believe we'll have solved the "simple" debate about the origins of the universe and life. For the first 2 years of college I thought I wanted to major in math... I might as well have chosen philosophy, because that's what math turned into for me. Try to prove zero and see what I mean. Yea, yea, calculus and all it's principles... still prove zero. The string theory maybe??

I'm kidding of course, but I don't think this one can be answered with what mankind knows today. Physicsman might not agree...
post #34 of 53
Quote:
Originally Posted by Martin Bell
Here's a couple of questions for general discussion - out of the two aforementioned forces, can one exist without the other? And when starting a turn, which one occurs first?
1) No, one can not exist without the other. As quoted earlier (3rd law of motion):
For every action there is an equal and opposite reaction

2) They occur simultaneously as they are consequences of each other per the 3rd law of motion.
post #35 of 53
We allow this to be so difficult. ;-)

A body must be "acted on" in order for it to be "accelerated" from it's steady state.
The resistance to the normal condition must come to play before any change will occur. A circle is a set of points on a plane all equal distance from a point.

We all need a session sitting behind the wheel of a racing go Kart. All 6 "forces of acceleration". Now that would be fun The Bears go karting!

All the confusion in this topic comes from the distance between our feet and our brain.

Our flexibility blurrs activity.(ref medmarkco above)

And Si is absolutely right! It is a matter of intent ;-)

CalG
post #36 of 53
Si

Would you care to state the "equal and opposite" reaction that is generated when a skier makes a turn to the north while traveling down an east facing slope?

CalG
post #37 of 53
North force acting on skier = -south force acting on snow/Earth. Note, because of the different masses involved, the Earth's acceleration is negligeable.
post #38 of 53
Oh Heck!

centrifugal force makes my ceiling fan swing around.
post #39 of 53
Talking from a technical point of view; centifugal force does not exist. The 'centrifugal force' that people often talk about is actually an apparent force, not a physical one. i.e. it is the reaction force to the one that creates the direction change.

Take for instance one of those rotating rides where you 'stick' to the wall, people often say 'centrifugal force' causes this, talking as though there is some force that pushs you away from the centre of the ride and back to the wall. This apparent force does not exist (i.e. there is no force pushing you straight back into the wall of the ride at all) the only components acting are the bodies tangential (or linear) momentum at a given instance, and the force applied by the wall to your body that moves you into a circular path (i.e. the centripital force). So really you have only two things to deal with pyhsically the bodies motion in a straightline and then the wall that exerts a force at right angles.

Long story short, centrifugal force is a name for an 'apparent' force, not a physical force and as such the name 'centrifugal force' is incorrect. This was basically a long ramble about not a lot () as the distinction makes little difference when discussing a physical situation but from a physics point of view it's totally incorrect.
post #40 of 53
If you want to explain the motion of a ball on your ride with respect to a point on the floor of the ride that is fixed at reference grid point (0,0) and use F=ma, you will need to include a centrifugal force in order to account for the appearent acceleration of the ball away from this point.

The centrifugal force is a tool that allows you to transfer your system of equations to an accelerating frame of reference, just like the Lorentz transforms allow you to transfer them to one moving close to the speed of light.
post #41 of 53
True, but if you were using momentum of a body and consider only a centriptal force acting you can resolve it without the need for 'centrifugal force' simply by dealing with the force-reaction pair.
post #42 of 53
except that the ball has no momentum in that frame of reference when your frame of reference itself is moving in a circle relative to the "stable" surface of the Earth.
post #43 of 53
i see what your saying, i was taking the view from the wrong frame. In that case then it can be used as a way to convert between frames (inertial/non-interial), good point.
post #44 of 53
Quote:
Originally Posted by No_Style
Take for instance one of those rotating rides where you 'stick' to the wall, people often say 'centrifugal force' causes this, talking as though there is some force that pushs you away from the centre of the ride and back to the wall. This apparent force does not exist (i.e. there is no force pushing you straight back into the wall of the ride at all)

CUT

Long story short, centrifugal force is a name for an 'apparent' force, not a physical force and as such the name 'centrifugal force' is incorrect
I like our carnival ride exmple so lets use it. I'm pretty sure you are talking about the one thats a rotating cylinder maybe 40 feet in diameter and 7 feet high and everybody stands along the inside of the outer curved wall - then the ride starts up - they darken the inside or flash some lights and what used to be the floor drops away from under your feet but youre fine and don't fall because you are pinned to the outside wall for some mysterious reason . As I recall, in most of these rides youre completely closed in so you dont feel any wind. Theres no way for you to see out so you dont really know for sure that you are rotating if you hadnt seen it running with the previous load of people before you got on. Besides - if you were blind you wouldnt have even had this little tidbit of information to help you figure out what is going on. At most a blind person might feel the rumble of the machinery but who knows exactly what that does.

For all the world it feels like something is pushing you against the wall - which now feels more like a floor than a wall. Youre curious about this force pushing on you so you stand up on this surface that has become a floor and start to do some experiments. You happen to have brought a scale along so you measure your "weight" and that of all the other people as well as whatever else is in your pocket. All the numbers are wrong but you happen to notice that they are all exactly equal to 3 times the normal weight of these objects. Hum - thats weird.

Next you start to drop the coins and marbles and other things that are in your pocket and measure how fast they drop. Amazing - they all drop at the same rate and the distance they fall is proportional to time squared - just like the do in the normal world but once again the proportionality constant is 3 times too large to be gravity.

So since I cant see out, cant feel any wind and have absolutely no direct evidence that Im going around in circles the theory I come up with is that for some mad scientist came up with a device in which gravity suddenly has been made stronger and now points at a surface that I used to think was a wall but now feels just like a floor.

So this new force that suddenly has turned on in his contraption seems pretty d-mn real to me and acts just like the force of gravity - only bigger - so Im calling it a force just like the force of gravity not some sort of a pseudo force or an apparent force.

Wind pushes my hand around when I stick it out the car window - I call that a force not an apparent force even though I cant see it.

Electrostatic force makes my hair stand up when I charge myself up. I call that a force not an apparent force evn though I cant see it either.

I can move steel washers around with a magnet. I call that a force not an apparent force even though I cant see whats causing the washers to move.

If it looks like a force, acts like a force and quacks like a force, Im calling it a force not an apparent force.

Thats my story and Im sticking to it.

YOT
post #45 of 53
I'd beware the wrath of PhysicsMan when making comments like "...centifugal force does not exist". He's faster than a given mass of lead accelerated by rapidly expanding gasses and travelling along a curvilinear path dictated by gravity - itself just a 'force' demonstrated by objects following a 'straight path' in a warped frame of reference (space)...

I kinda like the perspectives mentioned by YOT. There are lots of mysterious forces I can't easily see that I have to deal with. Like the unfathomable force that increases my weight ...over the course of the Thanksgiving Holiday.

.ma
post #46 of 53
No. You see the above mentioned are all physical forces, electrostatic force etc are all real forces. Centrifugal force is a convient idea, it's not a real or present force in the situation in which it is applied, it is simply a way of explaining a situation as was mentioned above you could apply it conceptually to transition to another frame of reference, however in a pure physics sense it is a ficticious force.

Look it up:

***
Centrifugal force
n. The apparent force, equal and opposite to the centripetal force, drawing a rotating body away from the center of rotation, caused by the inertia of the body.
***

There is no force that pushs an object outwards from a spinning object, the effect (or feeling in the case of the ride) that is normally attributed to 'centrifugal force' is caused ONLY by the bodies momentum (a linear quantity at any given instant) and the centripital force. The fact that you feel as though you are pushed backwards on such a ride is actually caused by the fact that you are pushed inwards at 90 degrees to your direction of motion by the wall against your back. Hence i damn well do mean 'centrifugal force' doesn't exist.

Heres a real explaination to save me having to explain myself properly:
http://www.madsci.org/posts/archives...1583.Ph.r.html

(Question was is the centrifugal force real)

Hi

If you are in a 'rotating' reference frame the force feels real enough.
Viewed from the outside, most problems dealing with rotation can be solved
in an inertial frame, i.e. non-accelerating/rotating. This schema has no
need of a concept like centrifugal force. The motion can be explained using
Newton's laws. Remember that 'real' depends on your motives. There is no
such thing as 'gravitational force' in General Relativity because gravity
is more accurately described as distortions in spacetime, not as a force.

I suppose the answer I would give to such a straight question is 'no, it
doesn't exist' because it can be artificially generated in the equations of
motion by transforming to a non-inertial coordinate system. A 'real' force
like electromagnetism has no such analogue except for a gauge transformation
(don't ask) but such a transformation is an abstract, mathematical one, not
one which we can directly experience like getting on a merry-go-round.

Just remember that 'forces' are just a language for describing nature which was
widely spoken in the classical era. Post- relativity and QM we choose to use
other languages (fields, transformations). What matters is whether the language
is a fruitful one. If you're trying to solve a problem of a man standing inside
a rotating box and kicking a football, the 'centrifugal force' is a very helpful
tool for getting out the right answer. However, if the ball goes outside the box
into a non-rotating room, it is an unnecessary complication and we just use
'straight' mechanics.

Hope this helps.
Cheers,
Max
post #47 of 53
I now have a headache. I tried swinging my security pass around but didn't get anywhere with that.
Will re-read Bob Barnes' explanation tonight, as I thought I understood that.
Always was crap at physics.
post #48 of 53
Quote:
Originally Posted by No_Style
-
No_Style, I think you are confused because of the use of the word fictitious. To me that word means "some human made up something that probably isnt true". In the case of the carnival ride nobody is making up anything untrue. No matter who measures the rate of fall of the marbles and how they do it or how accurately they do it theyll still get the same answer and just about every possible measurement that they do will seem to tell them that somebody temporarily cranked up gravity and pointed it in the wrong direction - at least as long as they are doing their measurements while along for the ride.

Im no physicist so maybe when those folk are talking to each other they need to have some word which says that forces like this are fundamentally different from other types of forces, but I think "apparent force" does justice to the situation much better than fictitioius or pseudo. This is a force which is apparent to anyone on the ride but it wont be apparent to someone standing still. A bunch of other physics geek websites used apprent instead of fictitioius.

I can tell you this - skiers midway aroud a turn are definitily along for the ride so they feel centrifugal force just like the guys doing the measurements on the carnival ride so in my old fogie opinion skiers wont care one hoot whether some physics website calls the force apparent or fictitioius - skiers feel it - its REAL apparent to them and thats all that matters on this discussion group.

YOT
post #49 of 53
true like i said at the start, this is a long ramble about not much

A ficticious force in physics is the same as what you consider an apparent force to be; more or less. Really come to think about this i didn't have much of a point to make considering this is a skiing forum, not sure why i laboured the point so much really
post #50 of 53
Nothing is real.

All forces, and indeed all sciences, are equally based on observations. The "electrical" force is based on observations of an attraction/repulsion, and an elaborate theory is built up to explain observations and make predictions. The electron, (or any other sub-atomic part if you prefer) is just as much an invention to explain and form part of a useful cohesive theory.

There is not absolute frame of reference! The forces that "exist" depend on your CHOICE of reference frame.
post #51 of 53
Re item 4: The reason "centrifugal force" is called a pseudo force is that there are only four identified forces in nature: electromagnetic, strong, weak and gravitational. While force is a measure of work, and if you were slung into the interior wall of a swerving vehicle you could certainly work the car or your shoulder, the force exerted is a derivative of inertia, calculated through Newtonian mechanics, a subset of the general theory of relativity, dealing with the force of gravity.

(One more angel dancing on the head of that pin.)
post #52 of 53
Excellent!
Now, tell us more about that gravity thing
post #53 of 53
Has anyone seen a gravitron? Or caught it and tamed it? Given it a name and trained it to be their ski buddy?
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