View Full Version : Math Formulas

Mike Olson
04-01-2014, 10:18 PM
Does anyone know of a website, or app, or something that would help me figure out what formula to use when designing a new project?

I'm picturing a site with some pictures of say a shelf supported by a bracket with x y z f shown and what formula to use to know how deep the fence could be for a set size bracket, and a max load, etc.
or a table with legs set in from the table top edge, and a formula to tell me if my 40lb daughter sat on the edge would it tip over? how far in could I place the legs? does the length of the legs matter? etc...

Then there is also, how much could a sheet of 3/4" plywood support without bending at what distance? x y z f. plug in the distance & thickness & weight and see if it can support it.
If I glued 2 sheets of plywood together does it actually double the weight it could support at the same distance?

or a wine bottle balancer, What are all the different forces in play? is there a formula that I could tweak the numbers to get the perfect alignment how I want it?

So many questions come up when i'm sitting here with a paper and pencil sketching out my project, but since I have no idea what the formula i need is called I have no way to google it. I don't even know if it's physics, or engineering, or Materials Science...

ugh, my head is spinning.

Jim A Walters
04-02-2014, 1:38 AM
For the plywood question you can look up "plywood deflection" and get a lot of info on how to calculate sagging and so on. There's a cool tool called the sagulator at the following web site that might be an easy way to calculate it based on the material and so on. There's other calculators at the same site.

For the table top I think you look at it as a lever and fulcrum. The fulcrum is the table leg nearest the edge where your daughter is sitting, the effort or applied force is the weight of your daughter, and the load is the weight of the rest of table from the fulcrum over to the far edge of the table including the far leg and anything sitting on the table. Then you measure the distances of the overhang of the edge. Look up "math formulas for lever and fulcrum" to get an idea of the calculations involved.

I can't quite picture the shelf, bracket and fence setup but I would think there would be info on calculating the weight a shelf will support if you look for that specifically.

I don't know of a particular web site or book that covers it all but maybe others might have some ideas there.

Hope this helps.


Steve Rozmiarek
04-02-2014, 9:07 AM
You are looking for center of gravity calculations.

Mike Olson
04-02-2014, 9:13 AM
Jim, That Sagulator is very interesting, thanks for the link. I'll be bookmarking that site. Lots of other good items in there as well.

Steve, Quick Google search on Center of Gravity Calculations gave me lots to look at. Thank you.

Montgomery Scott
04-02-2014, 10:11 AM
I use Roark's Formula's for Stress and Strain in addition to basic statics formulas. It will help you solve a plethora of different loading and constraint conditions. http://www.amazon.com/Roarks-Formulas-Stress-Strain-Warren/dp/007072542X

The amount of deflection for a beam is a function of its moment of inertia that is a function of the cube of its height so the doubling of the thickness of plywood, assuming a good bond to carry the interlaminar shear, will increase its stiffness by a factor of 8.

Bruce Pratt
04-02-2014, 10:16 AM
For a balancing wine bottle holder, it is difficult to make one which will take all shapes of bottles, the relevant bottle variables being at least bottle size/shape, glass thickness, neck length, neck diameter, and holder variables of angle of the base, base size, hole diameter, and hole orientation. You may get it to work perfectly for a bottle of Riesling, but it may not work for a Bordeaux. Your best bet is to use the curved design, although that also may not work for all bottle shapes.

Ryan Mooney
04-02-2014, 1:24 PM
If you can figure out how to ask the question properly https://www.wolframalpha.com/ can solve a lot of problems. Its figuring out the question :D And it also appears that they now want you to pay them a monthly subscription ("pro") to enable interactive use for most stuff.. bleh, i only need it like 3-4 times a year.

Mike Olson
04-02-2014, 1:42 PM
Ryan, that's a very interesting site. I'm poking around in the examples > physics section and finding lots of interesting things i may be able to use.

Thanks for the link.


Ryan Mooney
04-02-2014, 2:25 PM
Ryan, that's a very interesting site. I'm poking around in the examples > physics section and finding lots of interesting things i may be able to use.

Thanks for the link.


He's the same guy who wrote mathematica way back when.. Its definitely an interesting site - if you haven't found it the "examples" section is a good place to start (or spend a couple of days) http://www.wolframalpha.com/examples/

mike holden
04-02-2014, 3:31 PM
For a lot of good wood engineering questions, the US government put out a publication: "Wood as an Engineering Material" Almost 500 pages of good info, although presented in the dry language of a reference book. Best part is it is free. Available as a pdf file. Google it.

Mike Olson
04-03-2014, 8:02 AM
Thanks Mike, Chapter 9 has a lot of good formula but sadly, i need to look up about every other word.