Gravitational Tendency of Disks & Spheres

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In summary, the reason for larger scale objects, such as galaxies and solar systems, to have a gravitational tendency to form disks is due to the balance between centrifugal and gravitational forces. Gravity is not very strong and only becomes strong enough to make spherical objects at high densities or small sizes. The spherical halos in galaxies are composed of unorganized matter that will eventually settle into the disk. As for the universe, it is not rotating as a whole and its final shape, if there is one, is still unknown.
  • #1
Hippasos
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Hi!

Can someone illuminate why there are gravitational tendency in larger scale to form disks (galaxies, solar systems, accretion disks) whereas all smaller scale bodies (planets etc.) are spherical?
What about the spherical halos in the galaxies, is just "unorganized matter" which will "settle" in time and be part of the galaxy disk? How about the universe then? If it is not disk like or spherical, does it mean that universe is not rotating as a whole? Or is it because the universe is not in its final shape yet if there is one final shape ever to be?

Thanks!
 
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  • #2
Gravity isn't very strong.
To form a star or a planet you must have a lot of material pretty close together for gravity to pull it into a ball, the stars in the galactic halo are rather a long way apart and so the gravitational force on them isn't very strong.
 
  • #3
It comes down to the balance between centrifugal and gravitational forces. As mgb says, gravity is quite weak, but it goes as 1/r^2 so it only starts to become strong enough to make spheres at very high densities or small sizes.
 
  • #4
Galaxies and solar systems are disks because they form from a single rotating cloud of matter, and an object above or below the plane of rotation would not be as likely to have a stable orbit.
 

1. What is the difference between the gravitational tendency of disks and spheres?

The gravitational tendency of disks and spheres refers to the force of gravity exerted on objects due to their mass and distance from each other. The main difference between disks and spheres is their shape. A disk has a circular or elliptical shape, while a sphere is perfectly round. This difference in shape affects the distribution of mass, which in turn affects the gravitational tendency of the object.

2. How does the gravitational tendency of disks and spheres affect planetary motion?

The gravitational tendency of disks and spheres plays a crucial role in the motion of planets in our solar system. The Sun, which is a sphere, exerts a stronger gravitational force on planets than the planets themselves, which are mostly spherical. This force keeps the planets in orbit around the Sun and determines their distance from the Sun.

3. Can the gravitational tendency of disks and spheres change over time?

Yes, the gravitational tendency of disks and spheres can change over time. This is because the mass and distance of objects can change due to various factors such as collisions, accretion, and tidal forces. For example, as a planet grows in size by accreting material, its gravitational tendency will also increase.

4. How does the distribution of mass within a disk or sphere affect its gravitational tendency?

The distribution of mass within a disk or sphere can greatly influence its gravitational tendency. If the mass is evenly distributed, the gravitational force will be relatively uniform. However, if the mass is concentrated towards the center, the gravitational force will be stronger towards the center. This can be seen in the case of a black hole, where the mass is concentrated in a single point, resulting in a very strong gravitational pull.

5. Is there a limit to the gravitational tendency of disks and spheres?

Yes, there is a limit to the gravitational tendency of disks and spheres. This limit is known as the escape velocity, which is the minimum speed required for an object to escape the gravitational pull of another object. Beyond this velocity, an object will not be able to overcome the gravitational pull and will be pulled back towards the object. For example, the escape velocity for Earth is about 11 km/s.

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