Centre of Gravity

[blackeyed]

Why is the mass/weight of an object concentrated at its centre?

[ChuckFickens]

It's not.

[Old_Geezer]

Its centre ? At its centre of gravity. (Might be denser on one side than the other.)

Through where else would it act ? By definiton it's there.

[joggerjayne]

Surely "centre of gravity" refers to a point of balance, rather than a focal point of mass?

[Nibble]

If you mean that the mass of an atom is at it's centre then this is why....

http://en.wikipedia.org/wiki/Atomic_nucleus

[ChuckFickens]

Exactly, the centre of gravity would only be at the centre of an object if it was a perfectly uniform shape and density.

It's possible for the centre of gravity for an object to be outside the physical boundaries of the object (a horseshoe for example)

[factor-fiction]

What about a polo mint?

[joggerjayne]

"It's possible for the centre of gravity for an object to be outside the physical boundaries of the object"

Like roller blades. I'm sure the centre of gravity on roller blades is about three feet to one side.

[blackeyed]

lol @ polo mint
and yea i did mean it as an object of uniform shape and density.
I don't get why a uniform object get balanced at its centre when the mass is supposed to be spread throughout.

[Old_Geezer]

It's a mathmatical simplification. The mass is spread throughout. But it averages/cancels out and can be considered to be acting through the one point.

[jake-the-peg]

Imagine a thin wire with two identical weights at each end.

Balnced on your finger half way between

Got that?

Now Gravity exerts the same force on each the weight on the right wants to turn it clockwise, the weight on the left anticlockwise

The forces cancel out and it's balanced at the centre

Now imagine hundreds of them rotated around to make a disk

Now you have a point at the centre you can balance it on - the centre of mass - all those points are being pulled down and all balanced by similar points on the other side.

Move into 3D its the same principle - the force on one side is blanced with one on the other

[blackeyed]

if it is cancelling out an i imply that it would have odd number of atoms?

[Old_Geezer]

How accurate are your experiments ?
Maybe there are just 2 atoms, one either side.

[blackeyed]

thank you jake that made a lot of sense :)