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can someone check my maths

AndyP

AndyP

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Am I right it saying that if I wanted to cut a 3"x3"x3" cube from a sphere then the sphere would need to be a minimum 5.16" in diameter?
I have assumed that the longest diagonal within the cube (ie not one of the faces) must be equal to the diameter of the sphere.
Or to put it another way I cannot get a 3" cube from a sphere with a diameter of 4.75".
 
diagonal of one face = sq root of (3x3)+(3x3) = 4.24

corner to corner = root of (4.24 squared + 3 sq.) = 5.19

so yes not big enough.
 
I thought I'd try and answer the question by making a simple SketchUp drawing.

I drew your cube then drew two lines from opposite corners and where they crossed gave me the centre of the square. The measurement from that centre to the corner would give the radius of the sphere.

So I thought that while I was there, to prove my understanding was correct, I'd draw a circle through those points. However, when I tried to draw a circle centred on the centre of the cube, I could only get a circle in the red direction which wanted to snap to the midpoint of the side...rather than the corner where I tried to snap the circle to...but failed. Calling DaveR ..................help !!!
 
Hi Roger, you're on the right track. I can't get to SketchUp at the moment but try this. Use the Line tool to draw diagonal lines between the corners so you get a face that passes through the center of the cube. Delete the edges for one half of the cube. Make the remaining portion a component. Now you have a face onto which you can draw the circle. Draw the circle out dragging the radius to a corner on the cube. Select the edge of the circle and look at Entity Info for the radius.
 
Thanks, Dave. When I go to Entity information, it says the radius is approx 26mm (I used a cube of 30 x 30 mm). I seem to remember that one can set the number of decimal points on the tape measure but damned if I can find it!
 
Look under Window>Model Info>Units and set the Precision to the number of decimal places you want.
 
Dave R":38o6nctt said:
Look under Window>Model Info>Units and set the Precision to the number of decimal places you want.
Click to expand...


Thanks, Dave.
 
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