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Galileo's Diluted Gravity

  1. Oct 23, 2004 #1
    I am currently student teaching honors physics and while working on
    free fall a student asked me a question I could not fully answer. We were
    talking about Galileo's experiment using an incline plane and rolling balls
    in order to show that objects undergo uniform acceleration. I then discussed
    Galileo's thought experiment in which he tied a string to two stones with
    the same shape but different masses in order to show that acceleration
    is not proportional to mass. A student asked me how come,this was during
    a soapbox derby activity he had done in another class, heavier cars rolled
    down a hill faster than lighter ones. I was not sure how to answer that question. Is this due to rotational inertia??

    Thank you :redface:
  2. jcsd
  3. Oct 23, 2004 #2


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    The soapbox cars have to overcome the friction in their wheels (notice that the wheels are supplied by the organizers to equalize the friction burden). Galileo ignored other forces; that was his breakthrough, that had eluded previous thinkers. He was able to imagine the force of gravity separately from all the confounding forces and think up experiments in which it would be approximately freely exhibited.
  4. Oct 23, 2004 #3
    Air resistance for example does not depend on the mass of an object. So the same force acts on all the cars (assuming the same shape and velocity) independent of their mass. But this force wich opposes the motion of the cars has less effect on heavier cars, because of their greater inertia. So heaver cars are decelerated less by resistant forces thus acquiring a greater speed.
  5. Oct 26, 2004 #4
    Thanks for the info.

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