Orbital Speed of a Planet
Lesson notes
Overview
Planets farther out move more slowly: v ∝ 1/√r. This topic is rated intermediate and is number 69 of 98 in Earth & Space. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. 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
For a circular orbit, v = √(GM/r), so planets farther from the Sun move more slowly, v ∝ 1/√r. Mercury travels at 47.4 km/s, the Earth at 29.8 and Neptune at only 5.4 km/s. The orbit takes longer for two reasons: the path is longer and the speed is lower. This is also the content of Kepler’s third law, and it shows that gravity gets weaker with distance.
Reading the relationship
The expression "v = √(GM/r)" is a compact summary of the idea. It uses the symbols v, G, M, r; name what each one stands for before you start. Notice that it contains a square root, so the result grows more slowly than the quantity inside it, 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. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
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 “Ocean Acidification and pH”, the topic before it in Earth & Space. Once you are comfortable, continue with “Escape Velocity vs Planet Mass”.
Why it matters
Earth and space science helps us understand our planet, its climate and the wider universe.
Where you might use it
- Revising “Orbital Speed of a Planet” before a class, exam or certification
- Forecasting weather and understanding climate records and their uncertainty
- Planning for earthquakes, floods and other natural hazards in cities
- Interpreting satellite images, space missions and the age of rocks and stars
Frequently asked questions
What is Orbital Speed of a Planet?
Planets farther out move more slowly: v ∝ 1/√r. It is an intermediate topic in Earth & Space.
How do I use the Orbital Speed of a Planet simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression v = √(GM/r) packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Ocean Acidification and pH”, the previous topic in Earth & Space. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.