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making a weebley wobbly dog toy

 
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toymaker



Joined: 13 Jul 2009
Posts: 1
Location: USA

PostPosted: Mon Jul 13, 2009 9:28 pm    Post subject: making a weebley wobbly dog toy Reply with quote

Remember the old “weebles wobble but they don’t fall down” toys?

I want to make a toy that looks like a fire hydrant and is purchased on top of a sphere like base that is weighted and allows it to wobble like the old weebles and always come back to center standing balance. The top half of the fire hydrant is hollow and is filled with 8 ounces of dog food. There are openings at the top for the dog food to fall out of when the dog paws at the toy and tip it over.

I have a picture of a sample I had made on a SLA printer (let me know if you want to see it and where to send it). The base is currently ½ of a 6” sphere. When I fill the base up with water it weights about 1.75 lbs and does not come back to full standing center at rest after being tipped. When I fill the base with sand it weights 2.75 pounds and still does not come back to center at rest after tipping.

However when I fill the base with lead shot (the kind hunters use to make their own ammunition) it weights about 11 pounds and comes quickly and perfectly back to center at rest after being tipped.

Here is the problem. I want the design to work when filled with water, but I want the toy to be as small as possible. I know people with “physics smarts,” could quickly tell me the minimum size and shape I can get away with making the base, and using water (1 US gallon = 8.35 lbs) to allow me to put 8 ounces of dog food in the top and have it always return to full standing center at rest after being tipped.

Anyone out there willing to take this problem on?
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hepcj
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Joined: 23 Jun 2007
Posts: 125

PostPosted: Wed Nov 18, 2009 11:41 pm    Post subject: Reply with quote

The reason Weebles wobble but don't fall down is because they are designed so that when they are pushed to one side, their centre of mass is higher than when it is upright.
This requires that the base is not spherical but a flattened spherical shape. The object would return to its equilibrium even when made of a uniform density material.
When you start adding weight to top you raise the centre of mass.
Increasing the density of the material in the base would help. Lead would help but it is essentially the shape of the base that is the key here.
An interesting aside is the Gömböc which has only one stable position. If it is placed in any position other than its stable equilibrium position it will self right. It looks a bit like the shape of a tortoise shell.
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