Dot Physics

A surprisingly fun demo

You know I like demos, right? This one is quite fun to do even in a class. The basic idea is to take a bucket of water and swing it around in a circle over your head. Simple, but if you have never done this one, it can be a little intimidating. Here is an example.

Water Demo from Rhett Allain on Vimeo.

So, how does this work? What does it show? Really, the question is: why doesn’t the water fall out of the bucket?

First, I like to talk about “fall” what does that mean? I guess that means that the object has a downward gravitational force, but no force upward to give it a zero acceleration. Here is an example of a falling object.


In this case with the gravitational force as the only force, then the acceleration would be -9.8 m/s2. Thus falling

Now what about the water in the bucket? Here is a diagram while the bucket is over the head.


At it’s slowest speed, the water in a sense is falling. But this is different the the first ball in that the water is almost moving horizontally. You could do this with the ball also. Throw a ball over someone’s head. While over the head it is also falling, but it doesn’t hit.

Notice that I am trying to explain this so that you don’t need a lot of physics to understand it. The point is that the water is falling. Another important point is that gravity is still there. It is not being canceled or anything crazy like that.


  1. #1 Mike
    October 6, 2009

    I’m wondering where you get your cool looking free body diagram pictures from. Do you have some cool program that does that for you?

  2. #2 Rhett
    October 6, 2009


    I use Apple’s Keynote software. It is made for presentations, but it also makes nice simple drawings. I think use a screen capture program to take a picture of it. I remember some time ago seeing an online vector-based drawing program, but I can’t recall the details. If I do, I will let you know.

    Oh, if you have an apple computer, you can use the free trial version of keynote. I never save the drawings anyway.

  3. #3 Dave
    October 7, 2009

    I thought the demo about shooting bullets in the air was surprisingly fun! Err, what do you mean that wasn’t supposed to be a demo? Well, yes, the ceiling in the classroom did kind of get in the way.


  4. #4 Cleon Teunissen
    October 8, 2009

    About Vimeo:

    I seem to recall that in the past I could play Vimeo videos that were embedded in pages.
    But I’m not sure, maybe it was Youtube videos embedded in that were playable. I forget.

    What is supposed to happen if I go to the page ?

    Should the video start playing? Or should I maybe install software to enable playing a particular video format?

    The Vimeo site has help pages to assist users with uploading, but I did not find help pages for assistence with viewing Vimeo content.

    Cleon Teunissen

  5. #5 Rhett
    October 8, 2009


    For me, both vimeo and youtube videos are embedable and playable. I think they both use flash, so I would assume if one works, the other would work also.

    However (if my theory is correct) if it doesn’t work as an embedded video, it shouldn’t work on either. Maybe it is your browser or maybe you don’t have the latest version of flash.

  6. #6 Cleon Teunissen
    October 9, 2009

    I remember being able to play embedded flash videos, such as youtube and vimeo content, and my setup hasn’t changed. I wonder if Vimeo has reduced/changed its flash support, rendering my current setup ineffective.

    My apologies for bothering you with this. I haven’t looked what happens if I try another version of Flash player plugin. Possibly there has been redesign of Flash video format without backwards compatibility.

    Cleon Teunissen

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