Gravitational Field Strength
Field strength g = GM/r² at distance r from a mass.
About this topic
The gravitational field strength g is the force per kilogram at a point in space: g = GM/r². At Earth's surface g ≈ 9.8 N/kg, equivalent to 9.8 m/s² acceleration. It is a vector pointing toward the mass. The field idea says a mass changes the space around it, and any other mass placed there feels a force F = mg.
Moving further away weakens the field with the inverse square law. At the height of the space station, about 400 km up, g is still about 8.7 N/kg, roughly 90% of the surface value. On the Moon g is about 1.6 N/kg, and on Jupiter about 25 N/kg, which is why the same person would weigh very different amounts on different worlds while their mass stays the same.
Worked example: Mars has mass 6.4 × 10²³ kg and radius 3.4 × 10⁶ m, so g = (6.67 × 10⁻¹¹ × 6.4 × 10²³) / (3.4 × 10⁶)² ≈ 3.7 N/kg. A 60 kg person would weigh about 220 N there instead of 590 N on Earth.
Field lines are drawn pointing inward toward a mass, closer together where the field is stronger. Near the surface the field is nearly uniform, which is why projectile motion uses a constant g. Students often confuse mass, measured in kilograms and the same everywhere, with weight, a force that depends on local g.
Key ideas
- On Earth's surface g ≈ 9.8 N/kg
- Decreases with height
- Different planets have different g
Quick check
Use cases: where Gravitational Field Strength helps
- Homework: review the key idea behind Gravitational Field Strength before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Gravitational Field Strength without notes.
- Visual learning: play with the interactive lab to see how Gravitational Field Strength changes when you adjust its inputs.
- Teaching a friend: explain Gravitational Field Strength in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Gravitational Field Strength?
Field strength g = GM/r² at distance r from a mass.
What will I learn in Gravitational Field Strength?
On Earth's surface g ≈ 9.8 N/kg. Decreases with height. Different planets have different g.
Is Gravitational Field Strength part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Gravitational Field Strength lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Gravitational Field Strength?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.