How Gravity Works

Gravity is the force that pulls objects with mass toward each other. It's why apples fall from trees, why the Moon orbits Earth, and why galaxies hold together across millions of light-years. Understanding gravity helps us understand the fundamental structure of our universe.

The Basics of Gravitational Attraction

Every object with mass exerts a gravitational pull on every other object. The strength of this force depends on two things:

• Mass - More massive objects have stronger gravity
• Distance - Gravity weakens with distance (following an inverse square law)

This is why standing on Earth, you feel pulled down strongly, but you don't feel pulled toward your coffee mug (even though technically, you are - just incredibly weakly).

Orbital Mechanics

When an object moves sideways fast enough while falling toward a massive body, something magical happens: it keeps falling but never hits. This is an orbit.

The Moon is constantly "falling" toward Earth, but it's also moving sideways so fast that by the time it falls a little, the Earth has curved away beneath it. The result is a stable orbit that has lasted billions of years.

In Gravity Hands, you can see this principle in action. Particles near your hands don't just fall straight in - they acquire tangential (sideways) velocity that causes them to orbit and form rings.

Accretion Disks

When lots of particles orbit a central mass, they tend to flatten into a disk shape. This is called an accretion disk, and you can see them around:

• Black holes
• Newly forming stars
• Some planets (like Saturn's rings)
• Your hands in Gravity Hands!

The disk forms because particles collide with each other, averaging out their orbital tilts until they all orbit in roughly the same plane. The beautiful rings you see in the simulation are a simplified version of this cosmic phenomenon.

Why Particles Spiral Inward

In real accretion disks, friction between particles causes them to lose energy and slowly spiral inward. Material in the disk gradually falls toward the central object, which is why black holes can grow by "eating" their accretion disks.

In Gravity Hands, we simulate this with damping forces that slowly reduce particle velocity, causing a continuous flow from the outer regions through the disk and into the core.

The Plummer Model

Real gravity near a point mass becomes infinitely strong (a "singularity"). In simulations, this causes problems - particles can accelerate to unrealistic speeds when they get too close.

Gravity Hands uses a technique called Plummer softening, named after astronomer Henry Plummer. Instead of letting gravity become infinite at zero distance, we add a small "softening length" that keeps forces bounded. This produces smooth, stable physics without sacrificing the essential behavior of gravitational attraction.