Dot Physics

What is this #1: Answer

Perhaps you have had enough time to think about the first “What is this” demo item. Here is the item in question.


It seems most of you were right on track with this one – probably because you can still buy such a device. This thingy launches a small ball horizontally while at the same time dropping a ball. It is supposed to show that the vertical and horizontal components of an object in projectile motion are independent.

It takes two balls (which I didn’t have when I took the picture). One ball goes on each end and the arm is pulled back.


Update: Thanks to Kevin and Frank for pointing out that my diagram was backward – I fixed it.

Maybe this diagram helps. This particular model has two speed settings for the launched ball. Oh, the dropped ball has a hole in it so that it fits on that piston. When the piston pushes the launched ball forward, the ball drops off. Maybe I should have made a quick video. Oh well.

One last note – if you ramp this demo up to bullet speeds, it doesn’t theoretically work. The MythBusters did this and I showed why it wouldn’t work.

So, if you thought this first puzzle was easy, I think you are correct. Friday I will post number 2. Trust me, it is more difficult. Trust me.


  1. #1 Frank Noschese
    August 24, 2010

    I think you’ve got the dropped and launched balls switched. Have you used this before? It looks a bit dusty.

  2. #2 kevin
    August 24, 2010

    You have got the two labels reversed. When the arm is pulled back and locked under tension, the left side is where you would put the ball to be launched, and the right side (which is now poking out) the ball with hole to be dropped.


  3. #3 Rhett Allain
    August 24, 2010


    ARGGGGHHH YOu are right. I switched them. No, I have not used this particular device. We have a newer one that can be mounted on a ring stand – I use that one.

    Oh – I am fixing the image too.

  4. #4 charles bonyata
    January 10, 2011

    have a strange photo i took of markings on a pine tree. would like an opinion of what it may be. thanks

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