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Most UN-intuitive physics of everyday experience?

  1. May 5, 2007 #1
    What do you find to be the most UN-intuitive (non-intuitive?) of classical physics (pre-QM and Relativity)?

    Most people find quantum mechanics and relativity very UNinuitive, because they involve phenomenon at subatomic scales or very high speeds, both of which are well outside a human being's everyday experience.

    But there are some phenomenon that are part of our everyday experience that I find equally non-intuitive. Angular momentum, for example. Why a spinnng top shouldn't fall over just like it does when it's not spinning. Why applying force to the arm of a gyroscope results in movement at a right angle instead of directly in line with the force of the push. These seem very stange and difficult to understand and explain on a conceptual or intutive level.

    I'd love to hear what your votes would be for, and your best attempt to make them easier to understand.

  2. jcsd
  3. May 5, 2007 #2


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    Angular momentum is probably the most non-intuitive part for most people.
  4. May 5, 2007 #3
    The second principle of thermodynamics.
    Of course, its meaning for particular situations is always very clear and intuitive.
    For example, heat goes from hot to cold, very intuitive.

    But its formulation as a very general principle, "existence of entropy", is -by far- the most abstract statement of classical physics. I would even say the most abstract from the whole physics.

    This is not related to the simplicity or the complexity of the associated mathematics. The maths of thermodynamics are rather simple, specially when compared to electromagnetism, quantum mechanics or other domains. For example, in quantum mechanics, the concept of wavefunction is a kind of direct translation from experiences like photon counting and intereferences. A wavefunction is therefore a very concrete concept, easy to understand, although the maths may become tricky sometimes.

    The second principle covers such a broad range of phenomena and it is worded in such a general way, that it becomes more abstract than quantum mechanics. It is I think the most difficult concept of physics.
  5. May 5, 2007 #4


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    For me, I think that it's the concept of the finite yet unbounded universe. I simply cannot get my head around the idea of there being nothing 'outside' of it. I know that it's true, but there is no way that I can visualize it.
  6. May 5, 2007 #5
    For me the most unintuitive concept is that of forces in accelerating frames of reference. The forces on us due to the rotation of the earth, for example. I have no intuition here.

    There is a difference between being 'unbounded' and having 'no boundary'. If the universe was unbounded, then for any integer [tex] n [/tex] there would be two points in the universe and a reference from wherein the distance between the points is measured to be greater then [tex]n[/tex] (meters, light-years, etc).

    I would guess you have read, or someone has told you, that a finite sphere (the surface bounding a 3-dimensional ball) is of finite area but has no boundary (similarly, it distinguishes no point as its center).

    In fact this is true for the boundary of any finite shape (in any dimension), viz. that it is finite and has itself no boundary.
    Last edited: May 5, 2007
  7. May 5, 2007 #6


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    Hmmm... I've never even heard of that before. Yes, I meant 'without a boundary' as opposed to the other. That part isn't the problem for me, though, since I can understand it perfectly well for the inner or outer surface of something like a balloon or even a torus. It's that 'nothing outside' part that weirds me out.
  8. May 6, 2007 #7
    For me it is gyroscopic effect. You can hold in your hands and object that behaves differently as a "normal" object. It is not a concept; it is a material thing that seems to disobey your will.
  9. May 6, 2007 #8


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    Intuition grows as your experience grows. Most people don't start out with much intuition, and some of what they think they have is wrong.

    I guess a child that was born on a space station, who started playing with gyroscopes before it could walk, would grow up with pretty good intuition about both forces in rotating coordinate systems and gyros - but would find the environment on the surface of a planet very unintuitive (for example all the weird phenomema called "weather")

    From my own experience, after working on high speed rotating machines for a few years, accelerating frames of reference and gyroscopic effects did eventually become intuitive - and after that, it's hard to remember what it was like before that happened.

    On the other hand, I still think entropy is a type of magic best done by fluid dynamics guys at midnight when the moon is full (only joking!).
  10. May 6, 2007 #9
    hmm.. interaction between strong magnets is contrary to everyday experience.

    As a kid, the park had a giant metal climbing frame that spun on the central axis. It was easiest to start it spinning by running at the edge, but you could only reach the fastest speeds by running closer to the middle. You could sit (and hang on to it) and experience the centrifugal force, or you could have thown balls and watched the coriolis force's effect. Alas, they've since removed it.
  11. May 7, 2007 #10

    Andrew Mason

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    For me, an electromagnetic wave propagating through space without a medium may be the most unintuitive concept.

    It was also, apparently, quite unfathomable by 19th century physicists. Everyone was convinced that there was some medium called the "luminiferous ether". Michelson and Morley showed it was undetectable but few accepted that this meant that the ether did not exist. The Lorentz-Fitzgerald contractions were postulated in order to preserve this seemingly essential conceptual crutch.

    I don't understand string theory at all. But I would not be surprised if strings turn out to be simply a more sophisticated attempt to find an ether replacement.

  12. May 7, 2007 #11


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    I think it's just electricity in general. Everything else is pretty, well, easy to grasp for me. Everything about electromagneitcs is just so mystical and otherworldly.
  13. May 8, 2007 #12


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    On a uni-track vehicle, like a motorcycle, bicycle, or unicycle, steering left to create a lean to turn right and vise versa. It's not intuitive to focus on the lean and let the cornering take care of itself. This is best explained in web sites with articles about unicycles (two wheeled sites often implie gyroscopic forces at work, but these mostly just dampen the reaction rate).
  14. May 8, 2007 #13
    Gyroscopic forces are not dissipative and thus, they cannot dampen anything.
  15. May 9, 2007 #14


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    You're correct, it's too late for me to edit my original post, so I'll restate here that the gyrscopics forces increase lean stability, while slowing down (as opposed to dampen) the rate lean recovery that occurs from trail / caster effect (trail is the distances from where the front tire pivot axis intercept the pavement back to the actual contact patch where the front tire contacts the pavement).

    The steering geometry of a motorcycle is designed to cause the motorcycle to straighten up on its own if there is no force applied to the handlebars. Depending on the geometry (trail effect), there is a minimum speed where this self-correction will take place. As I mentioned, gyroscopic effect resists any change in lean angle, which I call a dampening effect.

    To lean a motorcycle over, a rider steers outwards to initiate a lean. The outwards steering can be the result of a rider applying an outwards steering torque force on the handlebars (counter-steering), or the result of the rider leaning inwards, causing the motorcycle to lean outwards (since the center of mass won't move sideways without a sideways force at the contact point between tires and pavement (or a side wind)), and the self-correcting geometry causes an outwards steering reaction to the motorcycle being leaned outwards.

    It's dangerous if a rider doesn't understand the concept of applying counter steering torque at the handlebars to lean a motorcycle. At lower speed, the motorcycle will self-correct in spite of body lean. At higher speeds, body leaning ceases to work.

    Getting back on topic, as speeds increase, gyroscopic reaction dominates, and there is less self-correction and more lean stability. At 100+mph, there's virtually no self-correction, just lean stability; a motorcycle will effectively hold it's current lean angle indefinitely. It takes the same (the very small difference is imperceptible to a human) amount of force to reduce lean angle as it does to increase lean angle, something that could be very dangerous to a rider not aware of how to use counter-steering to adjust lean angle, especially reducing lean angle to go straight again on a motorcycle. There was a quote regarding what it was like coming out of a turn onto the high speed banked section at the Daytona race track on a motorcycle, where the bikes are going 180mph to 200mph when they start to straighten up. The quote mentioned that a rider would either know about counter-steering, or would end up smashing into an infield wall.

    Anyway, the point of this, is that counter-steering on a motorcycle is counter intuitive. Motorcycle and bicycle sites seem to be random in the accuracy of how to steer a uni-track vehicle, but so far every uni-cycle site I've encountered descibes this process correctly (probably because at the speeds unicycles travel at, including effectively stopped while moving the wheel back and forth just a bit, there's no gyroscopic forces to confuse the issue). A web search for unicycle countersteering will provide a lot of good hits. Here is another good site:

    Last edited: May 9, 2007
  16. May 9, 2007 #15


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    Yet another motor vehicle related "un-intutive" driving method, driver induced understeer. Many people have probably experienced this, but few would think to use this as a spin recovery / prevention method while racing. Most racing schools don't teach this, since it's really only useful for cars with a loose (oversteer prone) setup. However, I've exchanged email with a few actual club and professional racers, and they do use this method for certain types of race cars, and some street cars: Acura NSX cars are tail happy depending on the setup, and the first year Audi TT's had a rear end lift problem at speed on top of a tail happy suspension setup, resulting in some bad accidents on the Autobahn (since then, TT's have a rear spoiler and a understeery suspension).

    Everyone is typically taught that you steer in the direction of the skid, called counter steering. Turns out that this is only true when accelerating, or at least enough throttle input to maintain speed. If a car is slowing down due to driver braking, counter-steering will make a spin even worse, since the front tires can use almost all their grip to apply a braking force that is inside the center of mass on a outwards yawed car in a turn. The same thing can be true if a car is slowing down due to engine braking (lift throttle), depending on how tail happy the car is. So here's the surprise, steering inwards even more can recover from or prevent a spin while braking or slowing down. What is happening is that the front tires are steered inwards past the point of maximum side force generation, causing the front end of a car to lose grip and wash out. This method will always work if combined with heavy braking. A race car may still end up going off track, but at least it will end up facing forwards instead of backwards.

    Since few people actually race in such cars, it's the racing game simulators where most have discovered this method. I made a video from a game to demonstrate driver induced undsteer, in most of the clips, I've exaggerated the inputs to make it clear what is happening.

    Here's the video:

    driver induced understeer.wmv
    Last edited: May 9, 2007
  17. May 9, 2007 #16
    "Dampening" has already a meaning in physics and in life. If you call "dampening" an opposing, compensating, resisting, etc, but conservative, force, you must not be surprised that people misunderstand you.
  18. May 9, 2007 #17


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    I corrected this abuse of the term dampning in the posts I could still edit, and added another post below to futher explain the corrections.
    Last edited: May 9, 2007
  19. May 9, 2007 #18


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    You are correct. I'll restate this: as speed increases, gyroscopic reaction increases lean stability (increases the tendency to hold a lean angle) and reduces (as opposed to "dampen") vertical stability (tendency for lean angle to remand at or return to vertical). As a consequence, it's not possible to body lean only, the equivalent of riding hands free, at very high speeds, 100+mph.

    After more research, it turns out that gyroscopic reaction doesn't make any significant contribution in speed wobble prevention or recovery. The key causes of speed wobble are insufficient trail (trail is the distance from where the pivot axis of the front tire would intercept the pavement back to the actual contact patch of where the front tire contacts the pavement), and excessive flex in the yaw axis of a motorcycle (frame flex, pivot points flex, which would allow the front or rear tire to move laterally or the rear tire to pivot) and tire sidewall stiffness. Adding a steering damper to resist pivot motion of the front tire can also reduce speed wobble, assuming the speed wobble is associated with the front tire and not a pivoting rear tire due to excessive play in the swing arm axis.

    As actual example, the first year of the Honda CBRR 900, there was insufficient speed wobble recovery at racing speeds, due to insufficient trail. The fix for that model was to add a steering damper. The fix in the next year model was to more the front fork tubes back by 3/8" to increase trail. Most motorcyles have sufficent trail and yaw rigidity that steering dampers aren't required. Still some racers will use them as "insurance" in case of a failure in a pivot point bearing, for example the rear swing arm axis.

    As another example, the Royal Enfield Atlas from the 1960's had excessive yaw related flex, and would speed wobble at speeds as low as 80mph to 85mph. I'm not sure how this was dealt with other than avoiding high speed.

    Sorry to get off topic, but I felt a corection to my previous posts was the responsible thing to do. As mentioned in a prevoius post, I myself was mis-led by a site that associated gyrocopic reaction with speed wobble stability, which isn't true as I've stated in this post.

    I now sit corrected.
    Last edited: May 9, 2007
  20. May 9, 2007 #19


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    To make up for my abuse of the term "dampen", and the incorrect association of gyrscopic reastion having any significant effect on speed wobble prevention / recovery, I'll include an interesting (to me and a few others) video of an aerobatic radio control helicopter in action.

    Keeping on topic here, what's "un-intuitve" about such models, is that they can pitch, roll, and yaw independently of linerar motion, bascially they are flying gyroscopes, only affected by the pull of gravity. In addition, with large thrust (power) to weight ratios, only a small compoent of available thrust needs to be downwards in order to resist the pull of gravity, so the main rotor can spend most of its time near vertical, with just short pulses of high thrust to counter gravity, while mostly accelerating the model horizontally for the brief periods of high thrust. For conventional flight maneuvers, thrust to weight ratio determines the maximum acceleration available for banked turns or loops. Getting back to the flying gyroscope comment, the cyclic, which varies the pitch of the blades as the main rotor rotates, is 90 degress out of phase with pilot input. Pitch thust is used to create roll reaction and vice versa. Yaw movement doesn't affect the main rotor (except for effective rotor speed), and is conventional.

    Here is the video, actual maneuvers start at 25 seconds into the video. Note that the first circling motion is done inverted and backwards, and the second circling motion late into the video is done inverted and sideways. The pilot is required to adjust his controls based on the orientation of the helicopter. Inverted, all the controls except roll are reversed. When flying sideways, pitch and roll control are swapped.

    aerobatic rc helicopter.wmv
    Last edited: May 9, 2007
  21. May 9, 2007 #20
    I agree, gyroscope effects are pretty weird, regardless of whether you look and them quantitatively or just qualitatively. I've seen the maths, but its still pretty odd the way that a spinning top seems to defy gravity.

    Magnets are still pretty strange to me too. I still don't understand what/why they do the things they do. It's all very odd.

    About vehicle mechanics - it's amazing how complicated that subject can be. But gyroscopic effect is probably still the strangest 'everyday experience'.
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