Gravitational fields and field strength
G3.3 G3.4
By the end of this lesson you should be able to
- Describe the concept of gravitational fields. G3.3
- Solve problems involving gravitational field strength at a distance from an object using g = F/m = GM/r^2. G3.4
Key ideas
- A gravitational field is the region around a mass where another mass experiences a force.
- Field strength g is the force per kilogram; its units N kg⁻¹ are the same as m s⁻².
- Field lines point towards the mass; closer lines mean a stronger field.
- g depends only on the source mass and the distance from its centre, not on the test mass.
Key formulas (QCAA formula book notation)
\[
g = \frac{F}{m} = \frac{G M}{r^2}
\]
Worked example
Find g at the surface of Mars (M = 6.42 × 10²³ kg, r = 3.39 × 10⁶ m).
- \(g = \frac{(6.67\times10^{-11})(6.42\times10^{23})}{(3.39\times10^6)^2} = 3.73\) N kg⁻¹
Common mistakes
- Using the mass of the object instead of the planet.
- Drawing field lines pointing away from the mass.
Do
- Read: Textbook (Oxford Physics for Queensland Units 3 & 4) section 5.3
- Do: CYL 5.3 Q3, 5, 7, 10
Try it
Watch
- Gravitational Field Introduction · Flipping Physics
Further reading
- OpenStax College Physics (AP) 6.5 Newton's universal law of gravitation (OpenStax, CC BY 4.0)
- Full topic notes: Gravity & Motion