5 ms·

Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense.

Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense. Anybody who has ever ridden a tilt-a-whirl instinctively knows there is a force pushing you out (otherwise you would go falling to the ground when it went vertical). Why science instructors are so focussed on the inwards force (in the case of a tilt-a-whirl, it's the outer frame, in the case of orbiting a planet/spinning neutron star, it's gravity) to the exclusion of everything else never made any sense to me, and caused me know end of distress. It probably came from the same place that suggested a charged item can attract a neutral object, but that a neutral object cannot attract a charged item. That made even less sense to me, but was right out of the book. Needless to say, my high school physics instructor and I never really got along very well.

EliRivers 12y ago
Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense. It makes an enormous amount of sense.
aptwebapps 12y ago
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.
- tjradcliffe 12y ago
  > 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.
  - aptwebapps 12y ago
    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.
  - ghshephard 12y ago
    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.
- EliRivers 12y ago
  There's also ordinary inertia which is what is pushing you Forces push. Inertia is not a force. Inertia doesn't push.
  - aptwebapps 12y ago
    Yeah, sloppy, sorry.
    - EliRivers 12y ago
      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 :)
      - ghshephard 12y ago
        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.
        EliRivers 12y ago
        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.
deleted 12y ago
[deleted]
snowwrestler 12y ago
Physics teachers are so focused on the inward force because everyone already knows about the outward force. In other words, the whole point of the lesson is to contradict what teenagers think they know. (Obviously, some teenagers don't react well to that, since teenagers think they already know everything.) The goal is to help students break through assumptions and understand that our everyday experience results from interactions of forces, not just singular forces themselves. A similar concept is the normal force. Everyone knows about gravity because they know what happens if they fall or drop something. But few people on their own will think of a force to explain why they are usually not falling. It's like the joke that "it's not the fall that kills you, it's hitting the ground." Analyzed with Newtonion physics, that is literally true! My physics teacher used to say "gravity never killed anyone," which is the sort of counter-intuitive thing that drives teenagers crazy. But it's true--it's that nasty normal force you have to watch out for.
- ghshephard 12y ago
  Right - I had no problem with the introduction of centripetal acceleration, in fact, after newton's laws, that made intuitive sense. It was the statement that "Centrifugal force does not exist" which upset me. Your explanation does a better job in a throwaway HN comment than he did in a formal physics class.

EliRivers 12y ago

Yeah, I get that he was teaching out of a book that told him (and probably every science teacher in existence) to teach this way - it just never made any sense. It makes an enormous amount of sense.

aptwebapps 12y ago

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.

tjradcliffe 12y ago
> 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.
- aptwebapps 12y ago
  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.
- ghshephard 12y ago
  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.
EliRivers 12y ago
There's also ordinary inertia which is what is pushing you Forces push. Inertia is not a force. Inertia doesn't push.
- aptwebapps 12y ago
  Yeah, sloppy, sorry.
  - EliRivers 12y ago
    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 :)
    - ghshephard 12y ago
      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.
      - EliRivers 12y ago
        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.

deleted 12y ago

[deleted]

snowwrestler 12y ago

Physics teachers are so focused on the inward force because everyone already knows about the outward force. In other words, the whole point of the lesson is to contradict what teenagers think they know. (Obviously, some teenagers don't react well to that, since teenagers think they already know everything.) The goal is to help students break through assumptions and understand that our everyday experience results from interactions of forces, not just singular forces themselves. A similar concept is the normal force. Everyone knows about gravity because they know what happens if they fall or drop something. But few people on their own will think of a force to explain why they are usually not falling. It's like the joke that "it's not the fall that kills you, it's hitting the ground." Analyzed with Newtonion physics, that is literally true! My physics teacher used to say "gravity never killed anyone," which is the sort of counter-intuitive thing that drives teenagers crazy. But it's true--it's that nasty normal force you have to watch out for.

ghshephard 12y ago
Right - I had no problem with the introduction of centripetal acceleration, in fact, after newton's laws, that made intuitive sense. It was the statement that "Centrifugal force does not exist" which upset me. Your explanation does a better job in a throwaway HN comment than he did in a formal physics class.