i About this experiment — click to learn the physics ▼
What you're looking at
Every pair of masses attracts each other through gravity. The top panel shows two bodies and the force pulling them together; the bottom panel is an energy diagram — the curved potential well U(r), the flat total-energy line E, and the gap between them, which is the kinetic energy.
Newton's law of gravitation
The attractive force between two masses grows with their masses and falls off as the square of the distance between them:
Halve the distance and the force quadruples — which is why the force arrows shoot up as the masses get close.
Gravitational potential energy
Moving two masses apart against their attraction stores energy. Taking the zero point at infinite separation, the gravitational potential energy is negative and gets deeper as they approach:
That's the bowl-shaped curve in the energy diagram. The force is just how steeply this curve slopes — a steeper well means a stronger pull.
The energy diagram
When you release the masses from rest, their total energy E is fixed (the flat line) at the value of U where they started. As they fall together, U drops into the well, and the freed energy appears as kinetic energy — the vertical gap KE = E − U. They rush together, are fastest at closest approach, then (after an elastic bounce at contact) coast back out to exactly where they began, the gap closing to zero. Total energy never changes.
Things to try
- Drag the masses close together and watch both the force and the depth of the well shoot up.
- Release from far away (shallow E line) vs close in, and compare how fast they end up moving.
- Make one mass much heavier and notice it barely moves — the light one does most of the falling (momentum is shared so the centre of mass stays put).