Universal Gravitation
Place masses, see gravitational field. Builds on Newton's Three Laws.
Imagine Earth were magically compressed to half its current radius, but kept the same mass.
What would happen to your weight on the surface?
Your weight quadruples! Newton's law of gravitation: F = GMm/r². Halving r means dividing by (0.5)² = 0.25, which multiplies the force by 4. The inverse square law means small changes in distance cause BIG changes in force.
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Place masses in space and watch the gravitational field lines appear. Move them closer and see the field strengthen dramatically.
F = GMm/r²
Setting GMm/r² = mg at Earth's surface gives g = GM/R². This connects the 'little g' you measure in a lab to the 'big G' that governs galaxies.
Tides are caused by the DIFFERENCE in the Moon's gravitational pull on the near vs. far side of Earth. The 1/r² law means even a small distance difference matters.
At a black hole's event horizon, gravity is so strong that not even light can escape. The 1/r² law taken to the extreme: r → 0 means F → ∞.
In 1798, Cavendish measured G using lead spheres — 'weighing the Earth.' The gravitational attraction between two 1 kg balls 1 m apart is only 6.7 × 10⁻¹¹ N. Tiny!
“Gravity is the weakest force but rules the universe. A fridge magnet beats Earth's gravity on a paperclip — yet gravity holds galaxies together. That's the power of being always attractive and infinitely ranged.”
Gravity Sandbox
Place masses in space and watch them attract each other. See gravitational field lines, potential energy contours, and watch orbits form.
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Place two equal masses and watch them collide. Now give one a sideways push — it orbits! Add a third mass and watch the beautiful chaos of the three-body problem.
Two bodies always have predictable orbits (Kepler's laws). Three bodies create chaos — no general solution exists. This is one of the oldest unsolved problems in physics.