Explain decreasing planetary orbital speeds farther from the Sun
Connect the Sun’s weaker distant field to the broad speed trend for nearly circular planetary orbits.
Section P6.1.2 · The Solar System
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Calculate average orbital speed.
- Recognise decreasing gravitational field with distance.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does the Sun’s field change with increasing Sun distance?
What you will learn
- Connect the Sun’s weaker distant field to the broad speed trend for nearly circular planetary orbits.
- Explain the distant-planet speed trend while distinguishing average and local comparisons.
Compare different orbital distances
The Sun’s gravitational field decreases as distance from the Sun increases. In the syllabus’s broad planetary comparison, planets farther away have lower orbital speeds and longer orbital periods. This compares different planetary paths, usually treated as approximately circular for the trend.
A larger orbit has more distance to travel, but it also takes much longer. Its period increase outweighs the radius increase in the average-speed calculation. Do not infer that the outer planet must be faster simply because its circumference is larger.
Distinguish cross-planet trends from one changing orbit
A supplied table can make the comparison explicit: model A at radius 1 unit and period 1 year has a speed factor r/T = 1; model B at radius 4 units and period 8 years has factor 0.5. The common 2π does not change their relative ranking. These are illustrative values chosen for consistent interpretation.
The Sun-distance field trend does not mean every instant of every eccentric orbit can be ranked by average radius alone. A single object in an ellipse changes its local speed along the path; the following energy lesson treats that separate comparison.
Sources for this lesson
Pause and explain
Which broad trend fits nearly circular planetary orbits farther from the Sun?
Worked example
Explain why an outer planet’s longer orbital path does not imply a greater average speed.
Show the worked solution
- Its orbital period is also longer.
- Average speed depends on path length divided by period.
- For the planetary trend, the period increase yields lower speed farther from the Sun.
Answer A longer path divided by a sufficiently longer period gives lower speed.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the field, orbital-speed and period trends for planets farther from the Sun.
Give me a hint
Treat all three as a comparison across planetary orbital distances.
Compare my reasoning
- The Sun’s field is weaker.
- Broad planetary orbital speeds decrease.
- Orbital periods increase.
Weaker field; lower speed; longer period.
Look for these in your work
- Identify Sun-relative distance.
- Include all three trends.
Compare model A with r/T = 1 unit/year and B with r/T = 0.5 unit/year. Find vB/vA.
Give me a hint
The shared 2π cancels in the average-speed ratio.
Compare my reasoning
- v is proportional to r/T.
- Divide 0.5 by 1.
- The result is 0.5.
vB/vA = 0.5.
Look for these in your work
- Use the period as well as radius.
- State a relative average speed.
A chart shows larger radius and lower average speed for an outer planet. A learner says this contradicts 2πr/T. Assess it.
Give me a hint
Check the period change before declaring a contradiction.
Compare my reasoning
- The circumference rises with radius.
- The orbital period can rise more strongly.
- The ratio 2πr/T can then fall.
There is no contradiction when the longer period outweighs the radius increase.
Look for these in your work
- Use both variables.
- Keep average speed distinct from local speed.
Common mistakes
- A longer period cannot be ignored.
- Compare distance from the Sun, not a planet’s own surface.
- A cross-planet average trend is not one ellipse’s local speed.
What can you explain now?
Which broad trend fits nearly circular planetary orbits farther from the Sun?
Compare with the explanation
Lower orbital speed and longer period
Outer planets experience the weaker field and traverse their larger orbits more slowly over longer periods.
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Make it stick
Explain the distant-planet speed trend while distinguishing average and local comparisons.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.