Surface Tension or Gravity?

Question

When a liquid is released in 0 gravity it takes the form of a sphere because of surface tension.
When rock, ice, dust, and other material clump together and form a planet it is called gravitational force.
What's the difference in the two forces?

Answer 1

The solids that you are talking about "find" each other in space because of gravitational attraction. Gravity is the weakest force in the universe, so for a droplet, surface tension dominates.

Aloha

Answer 2

Surface tension of a liquid is the reason you can float a razor blade on the surface of water. It's the reason that the edges of the liquid actually will hold the top of the liquid together when a glass is very slight overfilled and not be flat across the top of the glass, but rather convex.
It takes the shape of a sphere in your example because it is the shape of least distortion and, hence, the least differential forces acting on the droplet. In "mid evil" (pun intended) times, soldiers would drop a premeasured amount of molten steel or iron off the top of the castle down into the moat and later collect the approximately round cannon balls from the water.

Gravity, on the other hand, is a different entity altogether. It has nothing to do with surface tension and everything to do with distorting space and time. As they clump together, the objects will eventually form a sphere unless they are rotating, which will introduce outside differential forces (centripetal) which will tend to flatten the sphere at the polar regions and expand it in the center.

Good question.

Answer 3

Actually, solid matter starts to clump together because of tiny disproportions of electrical charge (this is also why dust sticks to things on earth). Eventually the clumps get large enough for the gravitational attraction to become significant. The clumps begin as very irregular shapes and do not look spherical until very late in their development.

The surface tension of water is also electrostatic in origin--water molecules are highly polar and tend to attract each other by the process of hydrogen bonding. The spherical shape results from all of the moecules crowding together in the most efficient way possible. The sphere minimizes the surfce area to volume ratio, so minimizes the number of molecules which are left unattached.

By contrast the electrostatic interaction among the various grains of dust is not homogeneous on the molecular scale, and the dust therefore forms more diffuse networks which (when they become large enough) then collapse under the influence of gravity.

Answer 4

Newtons law for gravity is as follows:

Fg=(m1*m2*k)/r^2

If you ran this through the gravitational force of a cup of liquid, or even a tanker car full of liquid, it would be a million times less than the surface tension of the liquid.

A clump of ice, rock and dust as a planet gets together, there is no surface tension, but the gravity is enough to pull all things equal distance from the center.

Go check out some asteroids or the moons of Mars (especially Phobos). You will see they are not large enough to become round from gravity. And they are hundreds of miles long.

Answer 5

Surface tension tries to minimize the surface area to volume ratio.

Gravity tries to minimize gravitational potential by maximizing the inverse square law.