Saturday, 4 December 2010

Up the slope

Can a double cone defy gravity?

Place the double cone on the lower end of the tracks and let it go.

The double cone seems to defy the gravitational force of the Earth and roll uphill.

The double cone paradox is an optical illusion, not a device that questions the laws of physics. Although the double cone appears to be rolling uphill, its centre of gravity is actually moving lower all of the time. There are three key variables at work here. The angle of the tracks in relation to one another, the angle of the tracks in relation to the horizontal plane, and the angle of the cones in relation to one another. The mutual relations of these variables determine whether the double cone will ‘roll uphill’ or not.

The first written notation about the double cone paradox and a detailed description of the experiment was included in the report written in 1694 by William Laybourn, an English land surveyor. In his report, William stated that although your senses can be deceived, the law of gravity cannot. This same logic remains true more than 300 years later.


Mathematics and history of the upphill roller


  1. Maggie Smith Puistola, 5 grade Puistola elementary school22 August 2011 at 05:50

    Does uphill roller work as good on the Moon as it does on the Earth?

  2. It does. The grafity is smaller in the Moon and there is no air (exept in a spacecraft landed on the Moon). Smaller grafity makes less fiction. Smaller friction and no air resistance makes the roller roll faster, but smaller gravity slower. The last change is a dominant one, so as a result upphill roller works on the Moon but slower than on the Earth.

  3. Thank you Timo. Our teacher thought, that upphill roller would work better in the Moon, because of less gravity. Obviously she was wrong.

    Maggie S.

  4. Hi Maggie

    Actually your teacher was neither wrong nor right in the physical meaning. No physical quantity has units on a scale bad - good or good - better - best. For instance the velocity of the roller is smaller on the Moon or it rolls slower. If you want you proper physical language, you dont say "velocity of the roller is slower", because velocity is physical quantity, which can get only nymerical values. They are smaller or bigger. Roller is a thing, which can move or roll slower or faster.

    Of course everybody understands you, if you say "velocity of the roller is slower". My point is, that if use logical physical language at least speaking about physics it is much easier to understand physics.

  5. Thank you Timo for your quick answer. It is rather hard to speak "physics", because I have studied physics only ½ year. How can I learn to speak "physics"?

  6. Thank you Maggie for your quick comment.

    I am physicist and I do not speak good "physics" in my every day talks. Not in english or in finnish. I might say that my weight is 80 kilos (well little over) when I should say that my mass is 84 kg. That would be honest and physical correct. So it is not neccesary easy to talk proper "physics", but worth of try.