Surface Gravity of the Planets

Lesson notes

Overview

Weight depends on a planet’s mass and radius; Jupiter’s gravity is 2.5 times Earth’s. This topic is rated intermediate and is number 31 of 98 in Earth & Space. Builds on the basics with relationships, methods and trade-offs.

What you are seeing

The panel above shows a bar chart that compares values on one scale, so differences in size are easy to see. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

The surface gravity of a planet is g = GM/r², which depends on both mass and radius. Jupiter’s is about 2.5 times the Earth’s, 24.8 m/s², Mars has 0.38 and the Moon 0.17, so a person weighing 70 kg on Earth would weigh the equivalent of about 175 kg on Jupiter and 27 kg on Mars. The mass stays the same everywhere. Surface gravity also influences which gases a planet can hold in its atmosphere.

Reading the relationship

The expression "g = GM/r²" is a compact summary of the idea. It uses the symbols g, G, M, r; name what each one stands for before you start. Notice that it contains a squared term, so doubling that quantity changes the result by a factor of four, and a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “Distances of the Planets from the Sun”, the topic before it in Earth & Space. Once you are comfortable, continue with “Kepler’s Third Law”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.