Dot Physics

More Punkin Chunkin Centrifugal Machine

This year’s episode of Punkin Chunkin is coming up (I think tomorrow). Discovery just showed a teaser commercial with the specifications for one team’s machine. If you are not familiar with Punkin Chunkin (World Championship Punkin Chunkin), the basic idea is to project some pumpkins. (note, if you waiting for the Discovery Channel show for the 2009 Punkin Chunkin, don’t click on the previous link, it has the results already).

One of the categories for Punkin Chunkin is the centrifugal machine. These are machines that spin pumpkins around really fast in circles to shoot them. They are basically like a giant-sized stone and sling. The one that caught my attention was the one from Greg Wolfe’s team Captain Inertia II. Here is a basic diagram of most of the centrifugal type machines.

i-ee00c82358969655282150fa817ad65d-cent_punkin_1.jpg

Really, the only reason for the centrifugal machine to move the pumpkin around in a circle is so that it can speed up over a longer time. The air powered launchers do this by having a longer barrel. There are a couple of issues though. In order to move something around in a circle (even at a constant speed) there needs to be a force on the object. The faster it goes, the greater this force needs to be. (Here is an introduction to objects moving in a circle). The relationship between the total force need and the speed and size of the circle is: (this is just the magnitude)

i-e9b1548f3ca4d8866125a06b17bb228f-fnet_circular.jpg

So, this is really the limit to the centrifugal machine. If the acceleration of the pumpkin is too high, it will lose its structural integrity (go splat). In a previous post, I estimated the maximum acceleration of a pumpkin. A typical compressed air launcher accelerates a pumpkin to about 600 mph in about 100 feet. This would be an acceleration of around 1000 m/s2.

For Captain Inertia II, what would be the circular acceleration?

Here is what I know. They claim a launch speed of 692 mph. I don’t know the radius of the circle, but he did claim that it will launch 80 feet above the ground. This would mean the radius of the machine would probably be less than 40 feet (not sure what angle he thought it would be released at). Let me just assume r = 40 feet = about 12 meters. Also, 692 mph would be 309 m/s. Putting this into the above formula and solving for Fnet/m, I get:

i-583ce47e89dd2252c63fe2a21804d3ff-circlular_accel1.jpg

Ok – there’s your problem. A couple of points. First, the machine may in fact be smaller than 40 feet radius. This would make the acceleration even larger. Second, the maximum acceleration of a pumpkin could be greater than this, but here the acceleration is way too large.

Well, how fast would it shoot at if it had a max acceleration of 1000 m/s2? Using the same expression and solving for the velocity, this would give a launch speed of just 110 m/s or 246 mph. Not good enough, not nearly good enough.

What about the range?

The other claim was that this launcher would shoot the pumpkin 1 mile. I have a suspicion that they calculated the launch speed that would give that range. Here is my short tutorial on projectile motion. This assumes that there is no air resistance, which clearly there would be some. If I use the numbers from the commercial and ignore air resistance, the pumpkin would go 27,000 feet when launched at 30 degree angle. Ok, then they did include air resistance.

I can’t tell exactly how they calculated the range for their speed. In my previous post, I estimated a pumpkin would have to be launched near 800 mph in order to achieve the mile range. I should probably go back and look at my calculations. It seems these guys might be using pumpkins that are a little more dense that I assumed.

Comments

  1. #1 Chris
    November 30, 2009

    I appreciate your analysis. But I don’t appreciate your move to a new server. In the old days (i.e. last week), I could read your blog in its entirety on Google Reader. Now, in order to read the post I’ve got to open a new window. Very sad for me, a physics teacher, because I love your work so much.

    Hopefully I can still get a lot from the first paragraph or two. Make ‘em really good for me, okay? Thanks.

  2. #2 Chris
    November 30, 2009

    About my last comment … I see from your next post that I can read that one all in google reader. So what’s the difference? Do you control that?

    I apologize for my earlier tone. Keep up the awesome analysis of everyday things. I do appreciate them very much.

  3. #3 chico mccarhty
    November 30, 2009

    mr wolfe design maybe improved releasing a counterweight that travels on a track curving out hitting the max. distance at the arc of the through release causing arm to whip projectiles with max speed, the only draw back is not destroying itself in the process. the counter weight wont have to be very big at speed the sudden stop of the weight may increase speed distance. let me know what you think.

  4. #4 Mike Hoy
    November 25, 2010

    How do the release mechanisms on these fast-spinning machines let the pumpkin go at exactly the right rotational position?

  5. #5 Mike
    January 2, 2011

    How do you time the release on centrifugal machine? the spin is way to fast for human effort. And what keeps the projectile in the breech until its optimum release. Almost the same question that Mike Hoy asked in November 2010
    Thanks
    Mike T.

  6. #6 Calibration Equipment
    March 17, 2011

    Well, how fast would it shoot at if it had a max acceleration of 1000 m/s2? Using the same expression and solving for the velocity, this would give a launch speed of just 110 m/s or 246 mph. Not good enough, not nearly good enough.

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