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📘 What is gravity?

On a lunar plain, a hammer and a feather hang side by side. Release them in a near-vacuum and they fall together: without air resistance, gravity gives objects of different mass the same downward acceleration.

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. The pull beneath your feetDistinguish gravity from air resistance and support forces, and explain why every mass participates in gravitational attraction.Gravity accelerates unsupported objects, while air and surfaces change what you observe and feel.
  2. Newton writes the ruleUse Newton's inverse-square law to connect mass, distance, G, and local gravitational acceleration.Newton's law makes attraction stronger with mass and weaker with squared distance; big G is universal while little g varies by place.
  3. Orbit is a kind of fallingExplain orbit and astronaut weightlessness as free fall, then connect gravity's long reach to cosmic structure.Orbiting bodies continually fall around one another, and gravity binds systems from moons to galaxy clusters.
  4. Einstein changes the pictureCompare Newtonian gravity with general relativity and identify lensing, clock shifts, and quantum gravity as consequences and frontiers.Einstein describes gravity as curved spacetime, explaining effects beyond Newton while leaving quantum gravity unresolved.

Questions this course answers

Why do a hammer and a feather fall together in a vacuum?

With almost no air resistance, their different shapes do not create the large drag difference seen on Earth, so gravity gives them the same acceleration.

What happens to gravitational force if distance doubles while the masses stay fixed?

Newton's inverse-square law divides the force by distance squared, so 2² leaves one-quarter.

Why do astronauts float aboard the International Space Station?

Earth's gravity remains strong there, but everything in the cabin follows the same orbiting fall.

How does general relativity describe gravity?

Einstein's theory treats free fall as motion through spacetime curved by mass and energy.

What can gravitational lensing reveal?

The distortion of background sources traces the gravitational field of foreground mass, whether luminous or dark.

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