Universal Gravitation
Describe gravity as a universal attractive force between all masses, explain that orbital motion arises because gravity provides the centripetal force keeping objects in orbit, and compare gravitational field strengths on different planets
What a learner can do afterwards
- States that gravity is a universal attractive force acting between all objects with mass
- Explains that orbital motion occurs because gravity continuously deflects the path of the orbiting object
- Compares gravitational field strength on different planets and explains how this affects weight
The lesson
Gravity isn't just Earth pulling you down. Every object with mass pulls on every other object with mass, even you and a friend standing near each other. That pull is usually too small to notice unless one of the objects is huge, like a planet, a moon, or a star.
Think about the Moon. Earth's gravity constantly pulls the Moon toward Earth's center. But the Moon is also moving sideways very fast. Those two motions combine into a curve, so the Moon keeps missing Earth and loops around it instead. It's the same idea as swinging a ball on a string: the string pulls the ball toward your hand the whole time, which is why the ball moves in a circle instead of flying off straight.
Your mass never changes no matter where you go, since it's just how much matter is in you. Your weight does change, because weight is how hard gravity pulls on that mass. That's why an astronaut who weighs 150 pounds on Earth would weigh only about 25 pounds on the Moon.
Gravity is a pull between every pair of masses, it bends orbiting objects into curved paths, and its strength changes from world to world, which is why weight changes even though mass stays the same.
Watch it
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.