If you tell someone there's no such thing as a centrifugal force and that (in rotation) there's only the centripetal force, you're half wrong. There's also ordinary inertia which is what is pushing you against the side of the tilt-a-whirl.
Telling someone there's no such thing as the centrifugal force in physics is a good starting point but you shouldn't stop there.
> Telling someone there's no such thing as the centrifugal force in physics is a good starting point but you shouldn't stop there.
No, it's a terrible starting point. Maybe the worst possible starting point. "Here is something you know to be true that I am going to tell yo is false, creating confusion and a tendency to reject anything else I say as obvious nonsense."
End with the notion of fictitious forces (which do exist, so the name is terrible), don't start with them, unless you want to be like one of those dreadful people who talks about negative generalized temperature or negative generalized resistance to neophytes without first explaining the generalization, so you deliberately mislead people into thinking you are talking about the familiar, ungeneralized concept.
There is no evidence whatsoever that such introductions make anyone more motivated or able to understand, and a good deal of reason to expect they don't.
Thanks very much - I think you are nailing it exactly. My physics instructor just blatantly said, "Centrifugal force - it's fiction. Doesn't exist. Centripetal acceleration does exist." He pretty much left it at that, so I tended to ignore everything else he said for the next couple years that didn't make sense to me, thought he didn't know what he was talking about.
I've probably learned more about the topic from this HN thread than I ever did in grade 11 physics.
Hm, that's a good point. I was over-generalizing from my own experience. Someone I respected started off that way, but he knew I would hang around for the full explanation and that I knew that he knew what he was talking about. It was just a hook.
Normally I'd not say anything, but this is a thread containing such gems as people divining the existence of forces on "instinct". There comes a point :)
But, the thing is, the centrifugal force does exist. It's a real thing, for the person in the tilt-a-whirl. Suggesting that instinct/observation is not an accurate method for identifying force does a disservice to observational science.
What was completely left out of the entire conversation in High School physics was the concept of frame of reference. By leaving out even a 5 minute briefing on that, my physics instructor lost all credibility (from me, and probably half the class), claiming that something that actually existed, did not exist.
It's a real thing, for the person in the tilt-a-whirl.
I disagree. It's not a real thing. If I pretend an acceleration that does exist doesn't exist, and then I realise that the only way I can make the mechanics work is by inventing a force to offset the force that's causing the acceleration I want to pretend doesn't exist, insisting that that invented force is "real" is just plain crazy.
It's a way to make the equations a bit easier. It's not real. There is nothing pushing anyone outwards. No atoms, no electromagnetic forces, nothing. There is no force pushing anyone away from the centre. To say that a force that simply does not exist, is "real", is just plain crazy or renders the word "real" meaningless.
Once again - I continue to get more value out of this HN thread than I ever did in high-school physics. You are distinguishing between a "real force" in terms of something that can be objectively measured irrespective of things like inertia, versus "real force" as measured as a function of the basic four-forces. What I think your saying, is just because a force "appears" in the equations in a rotating frame of reference, doesn't make it real.
Curious - which of the four forces are the source of the centripetal acceleration in a tilt-a-whirl?
Yes. I call a force "real" if there is a physical phenomenom causing it. An interaction. Electroweak, strong nuclear etc etc, or one of the convenient subdivisions that are often very helpful under various circumstances. This is what I mean by "real" force. If you have a force in your mathematical diagram that is not caused by one of these, it's not real. It's a mathematical dodge you're using for convenience of the maths, which is not by any means a bad thing, or wrong. It just doesn't mean that the invented force is real.
There is a wall behind the person, right? We don't use the phrase "tilt-a-whirl" here in the UK (or at least, in my part of the UK), but I think I know the device you mean. This sort of thing: https://debrasanborn.files.wordpress.com/2013/07/flat550x550...
That wall exerts a force on the person, inwards. That's the force causing the person to accelerate (i.e. change velocity).
What we conventionally label that force depends on where you are in the world, I expect, but it's the interaction of the person's atoms with the wall's atoms. Electromagnetic, or electroweak if you want to go more fundamental.
The person's atoms, of course, are exerting a force on the wall's atoms, outwards. That outward force is not exerted on the person. The person gets an inward force.
The force exists because the person's velocity is tangential to the circle. Sideways, if you will. But there is something in the way. The curving wall. The wall gets ever so slightly compressed (much as when you put a book on a table).
(I've simplified a bit - the person's velocity is pointing tangentially, and if the wall vanished they would zip off in that direction; the bit of the wall in the way is what they push on, and obviously there's not a single point of contact, so the forces aren't quite so simple as "a force" that way, and "a force" this way, but when they've all been summed, you get what we see - a force towards the centre. Also, weight still exists, and I've ignored that completely)
Thanks very much. Seriously - take this explanation and forward it to every high school physics instructor everywhere. Countless generations will sing your praise.
PS: Apparently they aren't called tilt-a-whirls, but RoundUps. And they are a lot of fun.
Telling someone there's no such thing as the centrifugal force in physics is a good starting point but you shouldn't stop there.