Pi Formulae Model

Pi Formulae Model

Pie belongs on the dining room table and Pi belongs among mathematical tables.

Both should be within easy reach.

It seems that every time I need to calculate a volume or surface area (and need to use Pi), I have to go and look up the formula. Well I decided to make a model showing the formulae at work.

I created a 'desk reference' model to have practical Pi formulae at my fingertips!

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In looking up the formulae for this model I found that when formulas were created “2π” must have been on sale. Besides the surface area of a cylinder formula: SA = 2π r² + 2π r h, there is also the surface area of a sphere: SA = 4π r² (4π = two 2π's?), so both formulae are heavy on 2π’s. In addition, I found that people like to write the formula for the circumference of a circle as C = 2π r. Again, using up those 2π’s.

Well, it’s not really two PI’s in the formula, it’s two times the radius multiplied by pi: C = π 2 r, AND…. 2 r is twice the radius which is actually the DIAMETER! So, I like to write the formula for the circumference of a circle as C = π d. Because when working in the real world I more often know or measure the diameter and not a radius. Sorry, formula gods, no 2π for diameters here.

[Note, that taking C = π d and solving for π gives you π = C/d, which is the definition of pi; the circumference divided by the diameter!]

Craft Snow Globe Container (Dollar Tree Store)

Empty Packing Tape Cardboard Tube

Pi Formulae Printed on Cardstock

Hot Glue Gun

Scissors

Cut out the formulae for Cylinders and wrap it around the base. Fasten the ends together and to the cardboard tube with hot glue.

Cut out the sphere formulae and bend it to fit inside the snow globe.

Run a bead of hot glue around the top edge of the carboard tube and glue the snow globe in place.

You can optionally tack the paper formulae disk in place inside the globe with a couple of dots of hot glue, use tape or leave it free-floating.

This is an optional step.

Cut out the base disk and hot glue it in place on the base cardboard tube.

Now anytime I need to calculate an area or volume for a project I have the formula at my fingertips.

No go out and Pi the World!

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Actually, the diameter of the earth is 7,917 miles (Seems kinda small, doesn't it?). Now go and see what you can calculate with that number!

Enjoy!