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Trying to recall an old experiment

  1. Sep 18, 2013 #1
    I have searched on google but cannot find this old physics experiment. It is most probably due to my failing memory and not choosing the right search terms I am hoping someone can recall the experiment.

    The experiment revolved around potential kinetic energy. it was designed to show that from a given height the potential energy of a mass was identical regardless of the path of descent.

    The way I recall the experiment was this.

    Two balls each of the same size and mass were positioned on two tracks. the tracks had different inclines.

    The balls were released together and although they proceeded down two different tracks they both passed the finish at exactly the same time. showing that both balls had the same kinetic energy at the bottom of the slopes. Most people would say the ball going down the steeper slope would gain more speed and therefore 'win'.

    If anyone can recall that experiment it seems to be a variation on Galileo’s Acceleration Experiment. having the two balls pass the finish line at the same time was very effective and it has stuck in my head.

    I'd love to know if there is a youtube video of the experiment I described. if not at least an explanation of how to set the experiment up.
  2. jcsd
  3. Sep 18, 2013 #2


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    The ball on the steeper incline will reach the bottom of that incline first. The speed of both balls will be the same after both reach the bottom of their inclines.
  4. Sep 18, 2013 #3


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    Time required to reach the bottom doesn't depend on the final kinetic energy, so you are confusing something. There is nothing strange in the fact different tracks require different times, actually finding the fastest track is an interesting problem, solved already in the 17th century: http://en.wikipedia.org/wiki/Brachistochrone_curve
  5. Sep 18, 2013 #4
    Thanks guys.

    Yes rcgldr, correct. This experiment I am talking about makes it very clear that regardless of the incline (excepting friction of course) the velocity of both balls will be identical when they reach the bottom of the incline. Therefore both will then race to the finish line in identical times.

    Borak, No I am not looking to find the fastest descent. I am trying to find an example of the experiment that shows the kinetic energy of both balls is identical at the bottom of the incline, when starting from the same height, irrespective of the slope.

    My fascination with this experiment is that it very vividly shows that the path the ball follows does not influence the kinetic energy. The main influence is simply the height or in other words, the potential energy both balls have at the beginning of the experiment.

    I was hoping to save myself some valuable time. I guess I could try and recreate the experiment using some old hot wheels tracks. Sadly we don't have any anymore since the kids grew up. Next time I am at my nephews perhaps.

    I think I remember now. Even though the tracks are of the same length the steeper track looked shorter than the track with the more gentle incline. Therefore intuition gets in the way and says the shorter looking track with the steeper incline will be faster. but its not.

    That's it! :) As a friend says. The fastest way between two points in the brain is via the mouth (or keyboard).

    Two chairs of identical height. Two tracks of identical length. Clamp the tracks to the chairs, make sure the tracks run parallel and the ends of the track line up. Move one chair closer to the ends (fixed so they don't move) than the other. This will give the track on the closer chair a steeper incline. This makes the track look shorter also the steeper incline makes you think the ball will travel down it faster, thus have more velocity.

    Position the balls on the tracks just where they leave the chairs. Release both balls at the same time. They both reach the other end of the tracks at the same time.

    I remember having to repeat the experiment multiple times to convince myself it was real.
  6. Sep 18, 2013 #5
    The shorter track is faster, if by faster you mean it takes the ball less time to travel it. The point is it doesn't matter how long it takes the ball to roll down the track, if they began from the same height they'll have the same speed at the bottom.

    You can easily measure the speed by letting them roll up an incline at the bottom, and they'll both roll to exactly the same height (assuming equal loss due to friction). Or, you could measure their momentum by allowing them to collide with something, and if they're the same mass measure the speed in that way.
  7. Sep 18, 2013 #6
    It's not possible to have two tracks of the same length at the same height but at different inclines - one has to be longer than the other.
  8. Sep 18, 2013 #7

    D H

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    The first statement is correct, assuming both balls roll without slipping. The second one is not. Just because the speed at the bottom of the ramp is the same does not mean the time taken to reach the bottom will be the same.
  9. Sep 18, 2013 #8


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    However, if you were to measure time it takes the ball to travel some horizontal distance AFTER the ball got to the bottom of the chair, this time would not depend on the initial incline. Hard to make this experiment with two balls going down at the same time (not easy to synchronize their arrival at the bottom, which is a real starting point), but measuring the time should be relatively easy.
  10. Sep 18, 2013 #9
    And I thought I had explained it so well. seems every one is on a different page to me. which has given me reason to doubt myself. Oh well when I get a chance I'll draw the layout. Sadly it also means the original experiment is lost in time unless I manage to recreate it.

    The main point i didnt explain properly is there is a fair portion of the track flat on the floor. The sloping portion is only at the start of the 'race'. Timing was easy just release both balls at the same time. You don't need to time when they reach the bottom.

    Of all the places to ask I though PF would most likely have someone who remembers the demonstration/experiment or even uses it with students.
    Last edited: Sep 18, 2013
  11. Sep 18, 2013 #10


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    What would be needed is some type of setup that would result in both balls reaching the bottom of the inclined planes at the same time.
  12. Sep 18, 2013 #11


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    Or let them run off the track onto carpet or something similar and measure the "winner" by which one rolls farther.
  13. Sep 19, 2013 #12
    Yes I think my memory of the demonstration is faulty. I know it was very good at showing the potential energy was reliant on the height not the path.

    If I can find or replicate the experiment I will post it for curiosities sake.
  14. Sep 19, 2013 #13
    Well, Galileo already demonstrated this by showing all objects accelerate at the same speed under gravity. Essentially he showed that (although it took Newton to formulate it in this way),

    [itex] \vec{F} = -mg\hat{y} [/itex] where [itex] g [/itex] was some constant acceleration. It's a trivial mathematical proof from there to show that you can represent [itex] \vec{F} [/itex] as the gradient of some potential, which implies that the work done by [itex] \vec{F} [/itex] is path independent.
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