The mysterious mysteriousness of simple physics

I was biking down the street and had to discard a banana peel. I approached a trash can and had to consciously decide to throw it at a point that seemed “too soon”–I was still several feet behind the receptacle. It went in.

What mystifies me is that the action was so unnatural. It really really felt like I should be throwing the banana peel just when I was going past the trash can. It was only my memories of physics class (and, I suppose, years of experience tossing things from a moving bike) that let me know when to release it–and, even then, it felt wrong.

I know that psychologists have done research on “folk physics”–the wrong intuitions we all have about heavy objects falling farther, ignorance of the action-and-reaction principle, and all the rest. But this thing with the banana peel is simple kinematics. You can’t get much more basic than that. Yet my intuition remains out of whack. What’s the deal?

15 thoughts on “The mysterious mysteriousness of simple physics

  1. Perhaps you were just never meant to be a Harlem Globetrotter?

    My guess is that your perception of velocity is altered after spending several minutes riding around. I know everything feels like it's going too slow when you get off the freeway after your brain has gotten used to everything going past at 70 mph. Perhaps you knew you were moving, but your brain was so used to moving at that speed that it felt as if you were standing still.

  2. My uninformed gut reaction is, we are used to __walking__ around and throwing things in trash receptacles all the time, making a compensation with our arms/hands to offset the walking. We do it without thinking. In a case where the velocity is just too high, either i) you'll naively miss the can; ii) you'll make a very unnatural adjustment and might make the can. But visually, the brain thinks, I am still just walking.

  3. It would be impressive if you waited until you were even with the receptical and threw it "backwards" with just enough force to make it go in.

  4. Wild guesses:

    Either: During the time that human thinking about these things evolved, human movement was almost always so slow that a "your own movement doesn't matter" heuristic was good enough. (They didn't have bikes, did they? And horseriding is recent, right?) At the same time, it seems it's not all that hard to train yourself out of that heuristic – basketballers have to throw accurately while moving all of the time. So it seems the human psyche remained flexible enough to unlearn the aforementioned heurisic for those who needed it.

    Or: The heuristic isn't hardwired at all, but you've acquired it because you don't usually move quickly when throwing stuff.

  5. These comments seem to ignore the fact that you did make the throw.

    If we assume that it wasn't a lucky throw, then your brain's perception of 1.) your velocity, 2.) the location of the garbage can, 3.) the weight of the banana, and 4.) the laws of physics were are all correct. Thus, your brain was able to execute a movement that adjusted for your velocity such that it could send the weight of the banana to the garbage can at the appropriate time with the appropriate force to get it into the can.

    The discrepancy is in what you 'thought' was going to happen and what really did happen. This thought is related to your prediction and evaluation of error which is dependent on visual information about the result of the action. Your brain had enough information to make the shot, but not enough to give yourself an accurate prediction of what would happen in the visual world, thus the dissonance you felt. This likely derives from the fact that you have little experience watching a banana (or similarly sized object) fly through the air into a can while riding a bike. Without this experience, it's an incredibly difficult thing to visualize (think of the difference between writing a computer algorithm for outputting a force and angle of throwing given a speed, a distance, and a weight, versus trying to depict this graphically over time). The visual system is our only route to getting information about failure or success in motor actions that involve objects leaving the body (we get some auditory information, but it's unlikely to be helpful in evaluating success).

    Your brain has no problem using physics in this case, it took what it knows about this novel situation and configured a motor action to match. It does this all day long. We very rarely execute the exact same motor action (think about all the degrees of freedom at play here). However, it does often see the same thing over and over again, so that's what it uses to predict error. Who cares if you sent the banana with a little extra speed this time compared to last as long as you still get it in the can (i.e., same physical/visual result). Until you get enough experience at visualizing the result and learn the visual result of certain actions, the action will 'feel' wrong, but be right.

    So, the mismatch is between the brain's ability to execute the action and your brain's visualization of the result of the executed action.

  6. You can conduct your own test. Attach something the size of a trashcan opening to a bike and have someone ride by on a bike while you try and toss a bannana peel to go into the trash can. Physically this is the same problem.

    My guess is this will feel more natural to you, because you see the trashcan moving and so you have to throw where you think it will be when the bannana peel gets there.

    Reminds me of when I tried juggling clubs on ice skates. When I dropped a club, I would just bend over and pick it up, because the club was sliding along the ice at the same speed I was. This felt very wrong, though when juggling clubs standing still, I expect the club to be where I dropped it.

  7. Couple of points not mentioned. On your bike, your inertial frame of reference is not stationary, so you are translating from your reference frame to another. That connects to the point above about being used to speed, but it is not merely a psychological impression.

    Second, the human mind is relatively incapable of grasping curving motions. This has been investigated in a number of contexts, perhaps most interestingly in the area of pick-pockets, con men and magicians; they rely on moving their hands in an arc rather than in straight lines. The explanation on one level seems to be that we are good at drawing a vector but that we interpret arcing motions by their tangents. We can draw the vector straight at a target but we draw a series of vectors tangential to a curving motion. The latter is a lot more mental work and tends to confuse our eyes and our minds, which as you know are somewhat but not entirely separate.

    So you are not only translating from your moving inertial reference frame but are doing that in a curve. That's why it may be better to use the Dwight Howard method and stuff it.

  8. Great post.

    The "heavy objects falling faster" intution is likely held because it is often valid in the realm of normal experience. Aristotle's theory actually is more accurate than Galileo's in a viscous medium. For many heavy-light object comparisons in daily life, air is non-trivially viscous.

    I suspect this kind of effect is true for numerous other such situations as well.

  9. If you throw sidearm (either across your body or as a "lateral") you can release as you pass the trashcan (assuming your not going faster than 20-25, which seems reasonable on a level surface in a non-race).

    The problem isn't that you needed to throw early to make certain it went in–the problem was that you needed to throw early to see that it went in because you worry about what you will have to do to 'fix' it if you miss. The 'problem' (counterintuition) comes from the extra risk aversion of the situation, not the situation itself.

  10. This reminds me of the helium-balloon in car trick. People have no problem with helium balloons going up when let go, but they are surprised that they will float forward when a car accelerates and foat to the inside of a turn.

  11. Being familiar with Andrew's love of both bananas and biking, I'm surprised tossing a peel in a can was "unnatural" for him. One of the common refrains from coaches is that a player's "thinking too much" (when the player should be playing "intuitively"). What you don't hear from coaches is "remember your physics" (they do say "focus on your mechanics", but it's a different sense of "mechanics").

    We're not surprised when we drop something while riding in a moving car and find it lands in our lap. Yet people don't expect dropping it out the window to have it land where the car will be. Part of the reason for that may also be friction — most of the stuff that gets dropped from moving vehicles has low weight and high resistance, and therefore lands behind you rather than next to you. Within a car, the windshield mitigates the effect of friction.

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