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Homework Help: Help on moption/forces project

  1. Mar 2, 2004 #1
    Hello All,
    I am helping my 3rd grade nephew on a science project. I am trying to decide what the expected results would be & why. The project is run of track that is inclined at both ends w/ a 30" straight run in the middle. The ends are 3" high at 30 degrees. He is releasing different size balls (golf, 1" plastic & marble) down the incline at one end. Then along the straight track and measuring how far up the incline on the other end the ball rolls. After five trials of each ball they all went the same distance up the incline.

    I know the math and physics behind this is way above 3rd grade, but want to understand it for myself and then give him a basic explanation of what is going on. I broke out my college physics books but was not able to puzzle through it.

    Any help is greatly appreciated. If I should post this to another forum I aplologize.

    Andy Murray
  2. jcsd
  3. Mar 3, 2004 #2


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    Conservation of energy. Every object starts with some potential energy due to its initial height. As it goes down, that potential energy is converted into kinetic energy. As it goes back up, that kinetic energy is converted back into potential energy. If there were no friction whatsoever, total energy would stay the same and the object (of any size, shape or composition) would rise to exactly the height it started from.

    Of course there is always some friction but it sounds like your mechanism is pretty good. You are probably getting little enough friction that all the balls are moving in the same way. (Conservation of energy- though not necessarily mentioned in those terms- is probably the point of the experiment.) Galileo's famous experiment dropping balls of different weight off the leaning tower of Pisa was basically the same experiment. (Actually, there is no evidence that Galileo actual dropped things from the leaning tower of Pisa. He mainly used inclined planes just like you are doing.)
  4. Mar 3, 2004 #3
    Thanks for the assistance. So I completely understand, without friction all the balls if started from the same place on the incline should raise to the same place on the end incline? Including friction, the heavier balls should go slightly lower then the lighter balls?

    Thanks again

    Andy Murray
  5. Mar 3, 2004 #4


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    I wouldn't necessarily say "heavier". Might be other things like "bigger so they make more contact with the track."
  6. Mar 3, 2004 #5


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    Or softer so there's more rolling resistance...
  7. Mar 3, 2004 #6


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    I think Nate has brought up an important point in explaining to your 3rd grade nephew. Find something other than a ball (maybe close to the same weight as the golf ball); an oblect that will slide instead of rolling to show that the area of contact and surface "roughness" causes friction (hence slows it down).

    Just an idea.
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