# spheres

• Jun 12th 2011, 09:29 PM
Veronica1999
spheres
Three mutually tangent spheres of radius 1 rest on a horizontal plane. A sphere of radius 2 rests on them. What is the distance from the plane to the top of the larger sphere?

The answer is 3+ root 69/3.
I really can't think of an approach, yet...
• Jun 12th 2011, 09:32 PM
Prove It
The larger sphere would never be able to rest on two smaller spheres without falling... This question = FAIL!
• Jun 12th 2011, 10:14 PM
earboth
Quote:

Originally Posted by Veronica1999
Three mutually tangent spheres of radius 1 rest on a horizontal plane. A sphere of radius 2 rests on them. What is the distance from the plane to the top of the larger sphere?

The answer is 3+ root 69/3.
I really can't think of an approach, yet...

1. The centers $(C_1, C_2, C_3)$of the smaller spheres form a isosceles triangle with the side-length 2.

2. The center M of the larger sphere is vertically above the centroid F of the isosceles triangle. The larger sphere is tangent to all three smaller spheres.

3. $MFC_1$ is a right triangle with
hypotenuse $|\overline{MC_1}| = 3$
horizontal leg $|\overline{FC_1}| = \frac 23 \cdot \sqrt{2^2-1^2}=\frac 23 \cdot \sqrt{3}$
vertical leg $|\overline{MF}| =\sqrt{3^2-\left(\frac 23 \cdot \sqrt{3} \right)^2} = \sqrt{\frac{23}{3}}$

4. You have to add one small radius and one large radius to the distance $|\overline{MF}|$ and you'll get the given result.

5. For additional information have a look here: Close-packing of spheres - Wikipedia, the free encyclopedia
• Jun 12th 2011, 10:21 PM
abhishekkgp
Quote:

Originally Posted by Prove It
The larger sphere would never be able to rest on two smaller spheres without falling... This question = FAIL!

some glue could be used. :)
• Jun 12th 2011, 11:34 PM
Prove It
Quote:

Originally Posted by abhishekkgp
some glue could be used. :)

It would still be top heavy and still fall...
• Jun 13th 2011, 07:47 AM
Veronica1999
Thank you so much Earboth!
• Jun 13th 2011, 01:13 PM
bjhopper
Re: spheres
also see Earboths solution of a four ball pile 12/22.10
• Jun 13th 2011, 05:08 PM
Veronica1999
Re: spheres
Thanks for the info. I really appreciate it.