Explain why planets orbit the massive Sun
Connect the Sun’s dominant mass to inward gravitational attraction and curved planetary motion.
Section P5.1 · The Solar System
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recognise gravitational attraction.
- Distinguish speed from motion direction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which object holds most of the Solar System’s mass?
What you will learn
- Connect the Sun’s dominant mass to inward gravitational attraction and curved planetary motion.
- Explain orbital curvature through gravity without adding tracks or forward engines.
Identify the source of the inward force
The Sun contains most of the Solar System’s mass. Its gravitational attraction provides the force that keeps planets following their Sun-centred orbital paths. A planet has motion along its path while its direction continually changes under inward gravitational attraction.
Gravity is not a second forward engine pushing the planet along its orbit. In a simple circular model the force points inward while motion is locally along the tangent. A force can change motion direction even when the speed is constant.
Use the dominant-mass approximation honestly
All masses attract each other, so a planet also attracts the Sun and the Sun is not perfectly stationary. The Sun’s dominant mass makes a nearly Sun-centred description useful for the syllabus. An equal-force interaction does not mean the Sun and planet have equal changes in motion.
If gravitational attraction were absent in a simple isolated model, the planet would no longer follow its curved orbital path. This explains the force’s role without adding a physical track, rope or continuous fuel supply. The model is qualitative and does not calculate an exact orbit from mass alone.
Sources for this lesson
Pause and explain
What force keeps a planet in its orbit around the Sun?
Worked example
A diagram adds a forward “gravity push” to a planet in a circular orbit. Correct its direction.
Show the worked solution
- Locate the Sun inside the orbit.
- Gravitational attraction points toward the Sun.
- The planet’s instantaneous motion is along the tangent, not along the inward force.
Answer Gravity points inward toward the Sun; motion is tangent to the path.
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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.
Explain why a nearly Sun-centred orbit model is useful for the planets.
Give me a hint
Connect mass distribution to the dominant gravitational source.
Compare my reasoning
- The Sun contains most of the system’s mass.
- Its attraction dominates the simplified planetary orbital model.
- Planets consequently follow nearly Sun-centred paths.
The Sun’s dominant mass supports the nearly Sun-centred description.
Look for these in your work
- Mention mass dominance.
- Identify gravitational attraction.
For a supplied circular orbital diagram, draw motion and gravitational-force directions at one point.
Give me a hint
One direction follows the path and the other points toward its centre.
Compare my reasoning
- Draw motion tangent to the orbit.
- Draw gravity inward toward the Sun.
- Keep the two arrows perpendicular in this circular model.
Tangential motion; inward gravitational force.
Look for these in your work
- Use the stated circular model.
- Keep force distinct from velocity.
A learner argues the Sun feels no force because its mass dominates. Correct this.
Give me a hint
Dominance does not remove mutual gravitational interaction.
Compare my reasoning
- The planet also attracts the Sun.
- The forces form a mutual interaction.
- Unequal masses produce unequal responses, so the Sun-centred approximation remains useful.
The Sun also experiences attraction; dominant mass does not mean zero force.
Look for these in your work
- Retain mutual attraction.
- Distinguish force equality from motion response.
Common mistakes
- Gravity is inward attraction, not a forward propulsion engine.
- Dominant Sun mass does not imply zero planetary attraction.
- Changing direction is a change in motion even at constant speed.
What can you explain now?
What force keeps a planet in its orbit around the Sun?
Compare with the explanation
The Sun’s gravitational attraction
Inward gravitational attraction changes the planet’s motion so it follows a curved orbital path.
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Make it stick
Explain orbital curvature through gravity without adding tracks or forward engines.
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.