Edit: In the time it took my meager brain to absorb Engineer and Ghost's posts above, and to compose the following reply, there have been 4 posts. Those even better than my post below, describe the conceptual battle that I am having. Thanks JASP and fom!.
Originally Posted by The Engineer
There most definitely is a force, and Ghost did a good job of explaining it. There's one more concept that you really have to wrap your head around that helps explain much of it. Entire fields of engineering live mostly to exploit this concept. Forces that are 90 degrees to each other do not interfere. With circular motion the centripetal force is always 90 degrees to the direction of travel, so even though there's always a force applied, the instantaneous linear speed doesn't change. Let's take an easy example of the earth going around the sun. The centripetal force is gravity. The centrifugal force in reaction to this comes from the earth's inertia. When you push on a rock, that rock will push back and accelerate away from you, but when you stop pushing that rock doesn't suddenly accelerate towards you. Similarly, when gravity is pulling on the earth, if it suddenly stopped pulling, the earth wouldn't accelerate in the opposite direction. What it would do is keep going the way it was going. It seems to me that some people think centrifugal force isn't real, because it doesn't accelerate you in the direction it's pointing when the centripetal force is removed. But, I can't think of an example of where that would happen for any reactionary force, so there's no reason to single out centrifugal force as different from other reactionary forces that people accept. When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force. When you stop pushing on a rock, it stops pushing on you immediately. The centripetal force is real, because it takes a force to change directions. The centrifugal force is real, because every force has an equal and opposite reaction force which you feel as pressure on your hands every time you push on something.
In the earth and sun example, the forces are easy to understand from gravity and inertia. In a train example (or skier), I agree it's harder to wrap the head around. Maybe an example to help understand it would be a ball bouncing off the ground at an angle. The only reason that ball can change directions is because it runs into the ground and the ground pushes back with a force from the impact (due to the balls inertia and the earth's inertia). When a train goes around a curve the rails push on the train to change it's direction. This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it. Otherwise, the train would just keep going in the same direction and fly off the tracks. If there wasn't a force there, the engineer wouldn't have to design the thickness of that lip to keep from breaking off, nor the thickness of the railroad ties to keep the the track from breaking apart. Using the speed and mass of the train, the engineer can calculate the centrifugal force that will be applied on those components to design them to handle the load. If those forces didn't exist, we could make those components out of something lighter to save money.
Predicting the future is pretty much all we do with trying to understand physics. When you understand why it's happening you can understand what will happen when the same conditions repeat in the future, because the laws are consistent. This is what we do with engineering. We predict that if the lip of that train wheel in the example above is greater than a certain thickness, it will not break in the future when the train comes around the curve at a certain speed. Or in skiing, if you apply a little more counter you can predict that .......
Engineer and Ghost:
"[T]here's no reason to single out centrifugal force as different from other reactionary forces that people accept. When the centripetal force is removed the centrifugal force immediately stops as well, because it was there only in reaction to the centripetal force."
If I accept their existence, I totally get that statement.
"This is why those train wheels have a lip that stick down below the rail, so that the rail can push on it. Otherwise, the train would just keep going in the same direction and fly off the tracks." You mean not in the same circular direction but in a straight-line inertial direction like the 8 ball being released from the centripetal push force of the crochet hoop, right?
I think the reason that the Doc guy in the video said that centrifugal force is a fraud and does not exist is because once the hoop was removed the centrifugal force disappeared. Which is what you are saying too I guess. You are saying the centrifugal force is merely a reactionary force to the actionary centripetal force and, as a result, you can't have one without the other. And once the centripetal force is gone, the centrifugal force also disappears. I get what you are saying there if I accept there existence.
"[C]entripetal force is real, because it takes a force to change directions." This is where, obviously, I have the problem. Having just learned about straight-line inertia I want to hold onto that concept like a baby blanket, and because I understand it, I want to use it to explain all manner of things. I will continue to mull this over. (And I am not an internet "Help Vampire."*) I help people for a living elsewhere and on the snow. I hate to frustrate you guys and gals, but in this thread it is all about me and my growth as a ski instructor.
So, here goes again, why must we call a wooden crochet hoop, a rock, a half-pipe wall, an uphill bump face, the surface of the Earth a "force" to explain what happens when an moving Mass encounters of object of greater Mass. The moving little Mass can't deflect, or move through, the larger immobile solid Mass, so the inertial direction of the more little Mass is merely redirected (the fom corollary) to a new straight-line inertial direction. If the larger immobile Mass is concave, the little Mass' direction will be redirected in a circular path.
But if the larger immobile Mass is convex - the direction of the little Mass will not follow the curve of the larger Mass. Rather, the little Mass' path of travel will be shot off into a straight-line inertial direction (into space, like me in the bumps).
It seems to me that the relative disparity of the Mass of the two objects sufficiently explains the physical result we observe.
Without having to resort to stating that the lifeless immobile wooden crochet hoop, rock, half-pipe wall, uphill bump face, or the surface of the Earth have some sort of mystical "force" to explain what is to my mind (and to fom) simple redirection of Mass by an object of greater Mass.
* Man there is a great discussion/warning of internet "Help Vampires" over at the Sportsmobileforum.com explaining that they frequent forums asking stupid questions and toying with the well-meaning helpful members and eventually suck the fun out of being a member of such forums. Resulting in the death of the victim forum by causing the most helpful members to flee.
Is that what happened to Bob Barnes?
** Engineer: Can I assume that meant to include the bracketed term in the following quote: "Similarly, when [Sun's] gravity is pulling on the earth,"
P.S. I am a Christian, so clearly I am capable of "faith." But intuitively, in my bones, heart, and soul, imbuing rocks, bumps, half-pipe walls and like objects with "force" is, at least at this point, a leap too far.