step 1 of 6
Weight changes from world to world because gravity belongs to every mass, not just to Earth. Even you and a friend pull on each other.
Why do you never feel your friend's gravity?
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
1 · Read
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. The pull is too small to notice unless one of the objects is huge, like a planet, a moon or a star. It also gets weaker as the objects get farther apart.
Try it together
Think about the Moon. Earth's gravity pulls it toward Earth's center all the time. But the Moon is also moving sideways very fast. Together these bend its path into a curve, so it keeps missing Earth and loops around it. A ball swung on a string does the same: the string pulls it toward your hand, so it circles instead of flying off.
So an orbit needs two things at once: a pull toward the big object, and fast sideways motion. An inward pull that keeps something circling is called a centripetal force. Take the pull away and the object carries straight on in the direction it was going. Take the sideways motion away and it falls straight in. Together they keep it falling and missing.
The numbers on the chart work as multipliers next to Earth's 1. Each one is that world's pull divided by Earth's pull. The Moon's 0.17 is about one sixth, and Jupiter's 2.5 is about two and a half times. To find what something would weigh on another world, multiply its Earth weight by that world's number. A helmet that weighs 30 newtons on Earth would weigh about 75 newtons on Jupiter and only about 5 newtons on the Moon.
Your mass never changes, wherever you go, because mass is how much matter is in you. Your weight does change, because weight is how hard gravity pulls on that mass. If you know a world's g in newtons per kilogram, weight = mass x g, just as on Earth. An astronaut carries the same mass to the Moon and back, while the pull there is about a sixth of Earth's.
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.
The rewritten readings were written for this dataset. Practice items x7 and x9 adapted from WeBWorK Open Problem Library problems by Stefan Lukits (British Columbia Institute of Technology) (CC BY-NC-SA 3.0). Practice items x1, x2, x3, x4, x5, x6, x8 and x10 are original to this dataset.
2 · Watch
3 · Play
Try it yourself
Fire faster and faster until falling turns into an orbit.
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
32 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.