Caption: A schematic diagram (called a free body diagram physics jargon showing the forces acting block on an wedge (or inclined plane): the forces being gravity W downward, the normal force N perpendicular to the contact surface, and friction F parallel to and up the incline at the contact surface.

Note the forces that act on the block NOT the forces the block exerts on its environment according to Newton's 3rd law of motion. If you try to include those forces, you create confusion with seemingly canceling forces.

Features:

1. This diagram can be used to explain why astronomical objects with sufficiently strong self-gravity are pulled into spheres ideally though NOT perfect spheres in real cases.

2. Now with friction, the block can be at rest. Without friction, it CANNOT.

   In a physics description, the 3 forces can add as vectors to zero, but without friction the 2 remaining forces CANNOT.

3. Now if the block and wedge were perfect fluids they would have absolutely NO viscosity (no "friction") and the block would just slide down the wedge.

   In fact, every bit of the block and wedge would slide and both would spread out into infinitely thin film on the horizontal surface.

   Real fluids have some viscosity and also surface tension (which is a force resisting spreading out), and so you don't get a an infinitely thin film when you try to build a hill of fluid, just a very thin film.

4. Another way of looking at the situation is that fluids have very low resistance to shearing forces.

   A pair of shearing forces are parallel by do NOT act along the same line. Thus, they tend to make layers of a body slide over each other.

   This is just what happens in fluids.

5. Now astronomical bodies can be made of all fluids (e.g., stars), but they can also be made or partially made of solids like most planets.

6. But if the self-gravity of a solid is sufficiently strong, the resistance shearing forces will be so weak that chemical bonds of the solid will eventually break sufficientl and the solid will act a physics plastic: i.e., it will flow: layers will slide over layers.

7. When will the flow stop?

   Only the pressure force is strong enough to resist sufficiently strong self-gravity. Atoms strongly resist being compressed.

   But note the pressure force does NOT resist shearing forces.

   So when the pressure force and gravity balance on each microscopic bit of matter flow can stop.

8. The self-consistent solution for just self-gravity and the pressure force is a spherically symmetric sphere.

   Gravity acts only toward the center.

   Each spherical shell has uniform pressure. So a radial pressure gradient balances gravity at every point radially. And there is NO tangential pressure gradient to cause a net tangential force.

9. If there is a centrifugal force due to rotation (relative to an inertial frame), the self-consistent solution becomes approximately an oblate spheroid.

10. Since almost all compact astro-bodies have rotation, most compact astro-bodies are approximately oblate spheroid with usually only a small amount of oblateness: e.g., stars and and planets.

11. How large does a astro-body have be to be pulled into nearly spherical shape?

    Well this depends on chemical composition, heat energy content, and rotation.

    However, observations suggest the empirical rule the size scale for a rocky astro-body must be >∼ 600 km and for a water ice astro-body must be >∼ 300 km (see Wikipedia: Dwarf planet: Hydrostatic equilibrium).

12. For a self-gravitating fluid body of UNIFORM density, the exact shape solutions are the Maclaurin spheriod and Jacobi ellipsoid.

13. Can the pressure force ever fail to balance self-gravity?

    General relativity (GR) predicts yes. A sufficiently compact massive object will collapse to being black hole. However, the interal structure of black holes is very uncertain.