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by edward morton on December 15th, 2005

edward morton

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Does a mass of solid material assume a spherical shape if subjected to equal pressure to all points on the object, and does the same thing happen if all pressure is removed, say in outer space?

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  • by Jeff Saxton on December 22nd, 2005

    Jeff Saxton

    External pressure:
    If a solid object is placed in a chamber of liquid and subjected to great pressure, it will not assume a spherical shape because no area of the object's surface is getting more pressure than any other. Pressure is still equal at all points on the surface.

    Of course if a solid object is forced into a spherical die or mold, it will assume that shape, because the points in contact with the die recieve more pressure than the "lower" points. The solid material is flowing like a liquid, but has to be forced to do so. It will stay that way unless someother unequal force causes it to flow again. Gravity could even make it flow and flatten against a surface if it's something like silly putty, or if it is heated until soft.

    In the weightless conditions of space, even silly putty (TM) would stay in a ball forever unless something comes along and smacks it.

    Gravity will force solids to assume a spherical shape, but only if the solid material can be made to flow by the object's gravity intensity.

    If an object is large enough in space, gravity will pull the material into a more or less spherical shape because all points are pulled toward the center of gravitational force and the object moves toward a state where it has the least surface area compared to it's volume. Because they don't have enough gravity to do this, asteroids and other small bodies in space look like potatoes or other odd shapes.

    On a smaller scale and with fluids, a force, usually called surface tension will do the same thing. Water forms a spherical shape when it "floats" it weightless conditions. Bubbles "want" to be round because of the forces pulling the "skin" molecules toward each other with equal force at all points.

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  • by purplecows on March 4th, 2010

    purplecows

    You are correct that zero pressure all around counts as equal pressure all around, but this has no influence on the shape of an object.

    Planets are spherical because gravity pulls down all the mountains above a certain height (and fills in all valleys deeper than a certain depth) depending on the strength of the material they're made of. So the total gravitational potential energy is minimized.

    Soap bubbles are spherical because of surface tension within the liquid, which tries to minimize the surface area while the volume of air trapped inside remains constant.

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  • by JakobA I^_^I the alooney on January 19th, 2010

    JakobA I^_^I the alooney

    No.

    External pressure does not change the shape of anything. It takes purposly directed external force to change the shape (eg: hit your grandpa's gold watch with a hammer enough times and you can mold it into a spherical lump)

    Internal forces however can change the shape of an object. among these are gravity (makes planets and stars round) and surface tension (makes raindrops round).

    So staying with grandpa's watch, you could get it to shape itself into a round gold ball by placing it in space and then heating it up above the melting point of gold. Then gravity and maybe surface tension (not sure, what is the surface tension of gold?) would pull it into a sphere.

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