Compare planetary gravity by mass and distance
Interpret surface field data and explain weakening gravity farther from one planet.
Section 6.1.2 · The Solar System
Core and Extended; Physics 0625, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked.
Before you begin
- Use weight = mass × field strength.
- Distinguish radius from altitude.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does gravitational field strength express?
What you will learn
- Interpret surface field data and explain weakening gravity farther from one planet.
- Compare mass and distance effects separately and preserve the limits of local uniform g.
State the comparison controls
The strength of a planet’s gravitational field depends on its mass and on distance from its centre. For matched centre distances, a more massive planet has a stronger field. Around one planet the field weakens as centre distance increases. These are different controlled comparisons.
A surface is itself at the planet’s radius from its centre. Comparing two planets’ surface fields therefore changes both mass and radius unless the radii are matched. “More mass always gives greater surface g” is not justified when surface distances also differ.
Keep field strength separate from force on a sample
Gravitational field strength g describes force per unit mass. A supplied local value lets you calculate weight using W = mg. Changing a test object’s mass changes its weight but does not change the stated planetary field value at that location.
Near a small region of a surface, problems may approximate g as uniform. That local approximation does not mean the field has the same strength everywhere around the planet. Moving significantly farther from the centre weakens it; use supplied values rather than assuming a universal surface g.
Pause and explain
For one planet, what happens to gravity farther from its centre?
Worked example
Two equal-radius planets have different masses. Which has stronger surface g, and why is equal radius relevant?
Show the worked solution
- Their surfaces are at matched centre distances.
- The more massive planet has stronger gravity at that distance.
- Equal radius controls the surface-distance factor.
Answer The more massive planet, because centre distance is matched.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Compare surface g for two equal-radius planets where B is more massive than A.
Give me a hint
Identify the controlled centre distance before comparing mass.
Compare my reasoning
- Equal radii give equal surface centre distances.
- B has greater mass.
- Its surface field is therefore stronger in this matched comparison.
B has stronger surface g.
Look for these in your work
- State the matched-radius control.
- Connect the mass difference to field strength.
A supplied surface model has g = 6 N/kg. Find the weight of a 4 kg object.
Give me a hint
Use the given local field value rather than Earth’s value.
Compare my reasoning
- Write W = mg.
- Substitute 4 × 6.
- Obtain 24 N.
24 N.
Look for these in your work
- Use the stated planetary g.
- Distinguish kilograms from newtons.
A table gives a more massive planet a weaker surface g than a smaller-mass planet. Is the table necessarily wrong?
Give me a hint
Ask whether their radii and surface centre distances are equal.
Compare my reasoning
- Surface distance can differ.
- Greater radius can reduce surface field.
- Mass alone is insufficient without radius or supplied measured g.
No; unequal radii can make mass-only ranking invalid.
Look for these in your work
- Identify the missing control.
- Avoid declaring a contradiction from mass alone.
Common mistakes
- Surface g depends on radius as well as mass.
- Test-object mass changes weight, not local g.
- A local uniform approximation is not a global uniform field.
What can you explain now?
For one planet, what happens to gravity farther from its centre?
Compare with the explanation
The field becomes weaker
Increasing centre distance from the same planet reduces its gravitational field strength.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare mass and distance effects separately and preserve the limits of local uniform g.
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.