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Skip to 0 minutes and 16 secondsSo we're here to reenact part of one of the great experience in science, the experiment that Galileo did to try and understand gravity. He started out with this simple kind of thought. If you have a heavy ball made of metal and a light ball made of wood, and you drop them, they land on the floor at the same time. Unfortunately, you saw that was very fast, and Galileo wanted to make the measurements and show that things really went that way, but it happened so quickly, they had no way to time that quickly.

Skip to 0 minutes and 47 secondsSo he came up with the great idea of using an inclined plane so that the gravity acceleration would be less, and therefore, able to be able to time it. He set up a very nice metronome, inverted pendulum, and he just measured how many strokes the pendulum went by for each distance that the ball went, and he found a very precise mathematical formulation, namely that in the first unit of time, if the ball went one unit on the inclined plane, the next unit of time it went three more units, the next unit of time it went five units, then seven, and so on in this nice progression. So we're going to reproduce part of the experiment where he made that measurement.

Skip to 1 minute and 27 secondsInstead of using a metronome, my assistants, Marie and Thierry, they're using smartphones with very good timers on them. And we will roll the ball down the inclined plane the way Galileo did, and we'll measure how long it takes to get to the first mark, and then the fourth mark. So we predict the time should be one unit here. It should be one more unit to get to the fourth mark. So are we ready? Ready. OK. I'll count down from three. Three, two, one, zero.

Skip to 2 minutes and 6 secondsOK. What were your times? 1.10 Very good. You're not cheating are you? This is the correct answer. All right. So now I need to find the ball. Thank you. OK, we'll do it again, because the essence of science is you repeat the experiment several times to see you're getting a consistent answer, and you checked out that you've made no mistakes. So three, two, one, zero.

Skip to 2 minutes and 45 secondsOK. 1.24. OK. Now we can just take the straight ratios, or we can just sum it up and divide. What do you have for the sum? 1.10 plus-- 1.24. 0.83, and 1.01. 4.18. That's the sum.

Skip to 3 minutes and 41 secondsOK. That's not bad. And we know from Galileo's careful measurements-- he used a very good inclined plane, very good timing-- that that ratio should be two, so we came out fairly close with this simple thing, and my little sloppiness in making it go.

Skip to 4 minutes and 3 secondsSo this is one of the things that was very interesting that Galileo did this; he did discover these things early, and then he moved on to other things. He was trialed by the church. They had at the 400th anniversary of this trial a special ceremony where they brought-- and a symposium-- where they brought out all of his publications, but it didn't include his publications on his early work. And he only published later after he was under house arrest, and he wanted to go and show that he really understood gravity and that the Earth really did go around the sun.

Skip to 4 minutes and 34 secondsAnd so he redid his early experiments and wrote them up really cleanly so that people like Newton and eventually Einstein could do it.

Experimenting with gravity

Nobel Prize winner George Smoot, with Marie and Thierry, reproduce with old-fashioned balls and board but modern stopwatches the inclined plane experiment of Galileo, which marked the foundation of modern physics.

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This video is from the free online course:

Gravity! The Big Bang, Black Holes and Gravitational Waves

Paris Diderot

Course highlights Get a taste of this course before you join:

  • Galileo and the falling bodies
    Galileo and the falling bodies

    Watch Pierre Binétruy explain how Galileo found the law of free, using a combination of real (inclined plane) and thought experiments (12 minutes)

  • First encounter with relativity
    First encounter with relativity

    Pierre Binétruy focuses on the notion of inertia, measured by mass, frame of reference and Galileo's principle of relativity