posted
What's the deal? Does it exist, or not? My physics teacher tried explaining it to me, but he's not very articulate. Anyone care to clear this up for me?
Posts: 1401 | Registered: Jun 2004
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posted
It does not, technically speaking, exist. It's a perception caused by Newton's 1st and 3rd laws.
A body in motion wants to go straight. It requires a force pulling in (centripetal) to make it turn. The body, as a result of newton's 3rd law, feels that force as a pull *out*.
So, if you hold on to a merry go round as it slings you around, your arm is pulling you towards the center of the merry go round, but you *feel* your body pulling out on your arm. What you *feel* is called centrifugal force, but only exists as a reaction to the force which is actually causing you to turn, just like you feel the force of the ground pushing up on your feet as a result of your body pushing down on the ground.
quote: What you *feel* is called centrifugal force, but only exists as a reaction to the force which is actually causing you to turn, just like you feel the force of the ground pushing up on your feet as a result of your body pushing down on the ground.
But why wouldn't the reaction to the force actually making me turn also be a force. Push and Pull are action and reaction, but they are both considered forces.
Posts: 1401 | Registered: Jun 2004
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posted
Depending on your frame of reference, you could say that either it exists or not.
It's hard to tell if a force is real or not. After all, how do you know that gravity exists and that the earth isn't just pushing against you?
For many applications, it's easier to do the math if you just assume that the force is real, and you still get the correct answer.
Of course, a man named Copernicus did a similar shortcut and assumed, to make the math easier, that the earth went around the sun instead ov visa versa, so YMMV. Posts: 16551 | Registered: Feb 2003
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posted
It's an acceleration. A change in velocity with respect to time. Since velocity is a vector--it runs in a straight line--if you are spun in a circle, your velocity is constantly changing direction. Constantly changing. Over time. Acceleration.
What yopu feel is a force. Acceleration multiplied by mass (which you've got) equals a force
A = dV/dt F = m*A = m*dV/dt
With velocity being a vector, you need to add a vector to the side to "deflect" it just a little bit, and eventually "wrap" it around the circle (or the curve). It's that contant "stream" of changing deflecting velocities (changing velocities) that make up the centrifugal force.
Or is that centripetal?
Posts: 1862 | Registered: Mar 2000
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posted
Inertia keeps you moving in the same direction unless a force (centripetal in this case) changes your direction or speed.
Often "centrifugal" force is attributed to inertia. But inertia isn't a force.
Bottom line -- no such thing as centrifugal force. The force points "inward" not "outward" Inertia makes you "need" to go outward.
Posts: 270 | Registered: Apr 2004
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so you understand that the ground doesn't push up on you except that you are pushing down on it, right?
it's the same thing with the centrifugal... it's only pushing because a force is acting on your body... take away the centripetal, and the centrifugal goes away. take away the weight, and the normal force goes away...
that's how you decide which one is real and which one is fictitious, basically.
Posts: 121 | Registered: Jan 2006
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posted
The thing is, gravity is just as fictitious as centrifugal force. If we were in the preferred reference frame, i.e. free fall, we would feel no force at all. The actual force on your body comes through your feet from the ground. It is the normal force. Gravity is just the word we use inside our particular reference frame for the force we feel that jams us down toward the floor.
This is very analogous to centrifugul/centripetal force, with: centrifugal force =~= gravity centripetal force =~= normal force from the ground straight line path =~= free fall curved path =~= fall being held back by the surface of a planet
(=~= is a symbol I invented that means "being analogous to".)
Relativity tells us that inside a closed elevator, there is no way to tell, using any instruments or measurement (that doesn't look outside the elevator) whether we are accelerating at 9.8 meters per second squared in free space, or sitting still on the earth.
Gravity = Acceleration. They are the same thing.
Posts: 6246 | Registered: Aug 2004
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posted
aspectre, you skipped the 20th century, so I thought I'd work that in between.
Einstein's paper about special relativity came out in 1905, I believe.
I've heard hints before about the effects of large rotating masses on time. It seems that time travel is possible that way, at least for elementary particle sized things. It's not Jane's inside-out ability to travel instantaneously with no energy needed to any point in spacetime yet, but it's very cool! Posts: 6246 | Registered: Aug 2004
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posted
Thanks a lot aspectre. I followed your second link and now my brain is trying to escape out my left ear. Posts: 26071 | Registered: Oct 2003
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We, on the surface of the earth go around on it. Our path, from the PoV of the center of the earth, is a circle.
The centripetal (center-seeking) force that keeps us moving in the circle is gravity. This force is real, as evidenced by the fact that we actually *do* move in a circle and return to that circle after, for example, jumping.
The normal force is purely reactive and, strictly speaking, centrifugal (it is directed *away* from the center of the circle), though it isn't "centrifugal" force as we normally think of it. It's a force that we perceive as a result of gravity. If gravity wasn't there, the normal force of sytanding on earth's surface would disappear. Remember that a fictitious force doesn't just mean it's "all in your head", though. Centrifugal force is real enough to cause a centrifuge to work or a pilot to black out, even if it's "fictitious".
Posts: 121 | Registered: Jan 2006
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posted
Password, no, you misunderstood me entirely. I was neglecting the effects of the earth's rotation totally and just explaining (obviously not very well) how it is that gravity is exactly as fictitious (no more or less) as centrifugal force, since it depends on which particular reference frame we choose.
Let me add to my analogy list. centrifugal force =~= gravity centripetal force =~= normal force from the ground straight line path =~= free fall or orbit, the path travelled by a light ray curved path =~= fall being held back by the surface of a planet
Note, this is an analogy. Pretend the planet isn't rotating. We're neglecting that.
Picture someone in the closed elevator feeling a force toward the floor. Is she accelerating or is she on the surface of a planet? There's no way for her to tell without looking outside, since gravity is the same thing as acceleration.
The thing is, that gravity is actually curvature of spacetime. So when you feel like you're sitting still on the surface of a planet, you're really deviating from the free path that follows the curvature of space. Those free paths are all in free-fall, either orbits around planets or drifting through space.
Another good way to picture it is to see a vast sheet of springy rubber, and a big basketball mass like the sun mashes it down so that smaller marble masses like the earth will, instead of following a straight line path rolling across the rubber, will curve inward toward the basketball or loop around and around it.
The curvature of spacetime is why gravity is so closely analogous with centrifugal force.
Sorry if my explanation sucks. I'm sort of braindead at the moment.
This is 20th c. physics. The stuff that aspectre linked to is more advanced than that. In school you're just expected to learn 17th or 18th century physics. If you bring up all this other stuff, your teacher may get annoyed at you.
posted
Centripetal force is the force keeping the object going in a circle from flying out as Newtonian Physics require. even though the objetc is flaying outwards the acual force is going toward the center of the circle.
Centrifugal force is the opposite force given by Newton's third law. However this instead of working on the object going around in circles it works on the center of the motion.
People often confuse centifugal force as the force that makes the object fly outwards when you let go (imagine a peice of string) however that is not centrifugal force but rather a lack of centripetal force that is causing this. Instead newtonian physics is taking over and making it contiue on in the velocity its going at when it loses the centripetal force.
Posts: 250 | Registered: Jun 2005
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posted
Hey the reason it was easier is that i just did an entire project on this and researched centripetal and cetrifugal forces and that is how i know all this stuff
Posts: 250 | Registered: Jun 2005
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