Built on Facts

Falling Down… Almost

Posted by Matt Springer on March 27, 2009
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Apologies for the light posting. Busy week, and it’s going to be a busy weekend too in physics land. Well, that’s what I signed up for.

This caught my eye though. Via Swans on Tea though, I see a great demonstration of physics as art. There’s a story of Feynman and several other physics lecturers who liked to attach a bowling ball to the high roof of their lecture halls and pull the ball out to an angle right by their faces and let it swing. Of course it comes almost back to hit them in the face,but not quite. The laws of nature just can’t provide more energy than went in to the system.

This is roughly the same principle, but with momentum. Conservation of momentum holds that the absence of external forces is equivalent to the statement that the velocity of the center of mass remains constant – stationary in many cases. Now there is an external force here – the forces of gravity and the normal force from the floor.


Machines that Almost Fall Over from Michael Kontopoulos on Vimeo.

The center of mass isn’t over the edge of the support when the hammer starts, and so it won’t be when the hammer hits – assuming the structure doesn’t deform and assuming you understand how the structure interacts with the floor. This artist does, and so the structures stay upright… barely!

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Comments

  1. #1 MikeMa
    March 27, 2009

    My intro physics prof did the bowling ball thing over 35 years ago. He also made a comment that if anyone in the audience would not consider doing the experiment him or herself, they should not plan to stay in science.

    I suspect some of them are now posting on Age of Autism. :-)

  2. #2 D. C. Sessions
    March 27, 2009

    I suspect some of them are now posting on Age of Autism. :-)

    Good thing ThinkPads have a fluid-catch keyboard tray.

    You got me.

  3. #3 Joshua Zelinsky
    March 27, 2009

    The bowling ball thing is also done in the book Contact with a pendulum. It is done as part of a larger discussion about evidence and faith. Of all the scenes that were cut out to make the movie, the removal of that scene annoyed me the most.

  4. #4 Ian
    March 27, 2009

    Richard Dawkins did this same stunt on one of his filmed lectures. I forget which one, but it was some years ago – some sort of memorial lecture if I recall.

  5. #5 qetzal
    March 27, 2009

    There’s a similar phenomenon that I find a bit more puzzling. Have you ever hit the top of a bottle and had it almost tip over, only to rock back the other way and then topple?

    Naively, you’d expect if it doesn’t have enough momentum to topple on the forward swing, it should have even less on the return, and therefore not topple. I guess it must be related to some interplay of forward and angular momentum.

    Has this been studied?

  6. #6 NoAstronomer
    March 27, 2009

    That is the coolest thing I’ve seen all week.

  7. #7 rob
    March 27, 2009

    i did the bowling ball thing for a demo during a lecture. it was fun. but not as much fun as the fire extinguisher powered cart.

  8. #8 Carl Brannen
    March 27, 2009

    “The center of mass isn’t over the edge of the support when the hammer starts, and so it won’t be when the hammer hits …”

    Good thing the qualifiers aren’t this week, LOL.

  9. #9 Matt Springer
    March 27, 2009

    Aw, cut me some slack Carl! The stuff after the elipsis might not be the clearest stuff in the world, but I’d still say it gets the idea across more-or-less correctly.

  10. #10 Jonathan Vos Post
    March 27, 2009

    I saw Feynman do that in the Bridge lecture hall. And Ed Stone after him, long before Ed was JPL Lab Director.

  11. #11 CCPhysicsit
    March 28, 2009

    Cool bit of art. I particularly like the lighting. Must think about stealing this idea and build one myself.

    I do that demo in my class, right in the middle of the lecture hall without a wall behind me. That means I have to really concentrate to avoid flinching, which is difficulty because we do a poor job of judging the acceleration of the ball. Closing my eyes only helps a little, because you can hear it coming. (Unlike most days in class, you can hear a pin drop when I do this demo!) I always end by giving the ball a push and then getting out of the way as it swings back higher than where it started, after explaining how important it is to release the ball rather than push it.

    PS – I would hate to see the performance of a group of grad students on that hammer device as a qualifier problem!