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Staiduk
2005-Feb-19, 08:34 AM
'Lo all!

Ok, this has bugged me for a long time: Centrifugal Force. It's constantly referred to as it's own 'force' as it were.

Er...isn't centrifugal force just inertia, or am I missing something? :)


Cheers!

lti
2005-Feb-19, 09:14 AM
technically centrifugal force isnt a force. it is, as u say, simply inertia. objects are apparently forced onto the outside of a spinning thing due to their tendancy to stay in a straight line.

for example. a bucket full of water on the end of a string being swung in a circle. the bucket is undergoing a change in velocity, thus it is experiencing an unbalanced force. this force is known as the centripetal force. in this case the force acting along the string causing the bucket to accelerate to the center of the circle that it is subscribing.

the water in the bucket experiences this unbalanced force accelerating it towards the centre of the circle, but its tendancy is to remain at a constant velocity (newtons laws of motion). it therefor remains in the bottom of the bucket.

laymen call this effect centrifugal force and say that is why the water remains in the bucket.

Grey
2005-Feb-19, 07:27 PM
Ok, this has bugged me for a long time: Centrifugal Force. It's constantly referred to as it's own 'force' as it were.

Er...isn't centrifugal force just inertia, or am I missing something? :)
Newton's laws of motion don't hold in a rotating reference frame. However, there are many times that it's really convenient to use a rotating reference frame (like, say, the surface of the Earth), and we'd like to be able to describe the motion of objects using something like Newton's laws. It turns out that if we add in a couple extra "force" terms into the equations (centrifugal force, based on your position, and Coriolis force, based on both position and velocity) the equations work out and we can still use them. So, yes, it is just a result of inertia and we could avoid invoking it if we restricted ourselves to nonrotating reference frames, but there are times that doing so isn't the simplest way to understand what's going on.

Chuck
2005-Feb-20, 01:59 AM
Of course it's a force. When I detect its carrier, the elusive centrifiton, I'll win a Nobel Prize.

mickal555
2005-Feb-20, 04:58 AM
In my bad astronomy book, Phil calls it a force so I posted about it- I was acussed of speaking hubbish though so I dunno if anyone understood me...

The Bad Astronomer
2005-Feb-20, 05:03 AM
It is a force... in a rotating frame. In a non-rotating frame, it's a product of inertia.

mickal555
2005-Feb-20, 05:04 AM
ohhhhh :oops:
OK, sorry

Hi! btw..

Staiduk
2005-Feb-20, 06:38 AM
It is a force... in a rotating frame. In a non-rotating frame, it's a product of inertia.

Hey; thanks all. I thought I had it down, then Dr. Phil dropped this one in, and I'm confused again. #-o :lol:

OK; let's see if I can puzzle this one out.

(And yes; I know right now there are a whole lotta astronomy ans physics types going "Awwwww - look at the widdle soldier trying to think...isn't that cuuuuuute??? :D :D :D)

OK - in a non-rotating frame - by that I assume to mean a region effectively devoid of movement - if we're watching a spinning object such as the stereotypical pail of water on a string, we can see that the force pulling the bucket (and water) outward is simply inertia.
BUT if we're observing it from the inside - i.e. standing in that bucket of water (big bucket...), we're in a rotating plane of reference; therefore from that vantage point centrifugal force would be recognized as a viable force of its own.

Er....right? 8-[

OK, now for the really chintzy bit - that was leading up to my next question; which is centripetal force. From what I've seen, it has even less call to be considered an independant force than its opposite - to me it's either tension or gravity; depending on what's keeping the rotating mass from flying off. That seems to hold true whether one is inside or outside the rotating frame; so what am I missing here? :)

Thanks! (For confusing the heck out of me, that is. ;) ;) )

W.F. Tomba
2005-Feb-20, 06:52 AM
OK, now for the really chintzy bit - that was leading up to my next question; which is centripetal force. From what I've seen, it has even less call to be considered an independant force than its opposite - to me it's either tension or gravity; depending on what's keeping the rotating mass from flying off. That seems to hold true whether one is inside or outside the rotating frame; so what am I missing here? :)
You're not missing anything, but you may be misunderstanding terminology a little. Centripetal just refers to direction; it means "toward the center." A centripetal force is simply any force that accelerates something toward a central point.

Jpax2003
2005-Feb-20, 07:04 AM
I sometimes get confused too. I just try to forget the meaning of the word and just look at the math. But someone once tried to illustrate it like this: Centrifugal is like a weight on a string moving around a point. Centrepetal is like a ball moving along a round track like a roulette wheel. I know the analogies are slippery, I'm just not sure where they slip.

Donnie B.
2005-Feb-20, 02:33 PM
My understanding is that "centrifugal force" refers to the tendency of an object in circular motion to try to fly off tangentially to that motion. It's the force that seems to push us to the right as our car takes a sharp left turn. As Phil implies, if we observe this force from a non-rotating perspective, we can see that it is a result of inertia. If we're rotating along with the object, we could (relativistically speaking) call the force "gravity".

Don't forget, though, that relativity allows us to think of these "forces" as space-time curvatures.

"Centripetal force" refers to any force that is preventing the object in circular motion from flying off tangentially. In the ball-on-a-string example, it would be the tension in the string, which is directed toward the hand that's doing the flinging. In the case of a planet orbiting a star, the centripetal force is gravity. In the turning car, it's the friction between the seat and our posterior anatomy.

Kristophe
2005-Feb-20, 05:15 PM
Let's see if I remember how this works...

Ok, you're holding a piece of string with a ball attached to the end. You start to spin in a circle, and find that the ball moves away from you intill the string is taut. From your point of view, something is pulling the ball away from you. By holding onto the string, you're applying a centripital force to the ball. But the ball isn't moving toward you. You see it in equilibrium, and since you don't have the advantage of being in two reference frames at the same time, you interpret this as meaning there must be some centrifugal force pulling the ball away from you.

I hated my second year dynamics course, though, so this is really hazy to me.

A Thousand Pardons
2005-Feb-20, 08:43 PM
laymen call this effect centrifugal force
and a few professionals (http://www.badastronomy.com/phpBB/viewtopic.php?p=29784#29784) :)

jfribrg
2005-Feb-21, 06:51 PM
Here (http://www.badastronomy.com/phpBB/viewtopic.php?p=331275&highlight=centrifugal+force +schaums#331275) is a discussion from long ago concerning this issue.