A gravitational field is a region where a mass feels a force. Its strength is the force per unit mass, and the force between two masses follows an inverse-square law.
A gravitational field is a region where a mass experiences a force; its strength is g = F / m. Between two point masses the attraction is F = Gm₁m₂ / r², with uniform spheres treated as point masses at their centres.
A mass placed near a planet feels a pull even with nothing touching it. That is a field. Its strength at a point is the force on each kilogram, g = F/m, and the simulation shows how the force between two masses grows with each mass and falls with the square of their separation.
Newton's law of gravitation gives the attraction between two point masses as F = Gm₁m₂ / r², directed along the line joining them. A uniform sphere behaves exactly as if all its mass were concentrated at its centre, so for planets and stars r is the centre-to-centre distance. Field lines drawn toward the mass show the direction of this force, and their spreading shows the field weakening with distance.
Four quick checks on field strength, field lines and Newton's law. Each correct answer earns XP and lights this skill on your star map.
A gravitational field strength of 9.8 N kg⁻¹ at a point means that:
The gravitational field lines around an isolated point mass:
In F = Gm₁m₂ / r² applied to two uniform spheres, the distance r is measured between:
Two point masses attract with force F. If both masses are doubled while the separation is unchanged, the force becomes:
Treat uniform spheres as point masses at their centres, so use the centre-to-centre distance, never the gap between surfaces and never the radius. And keep the two ideas separate: field strength g = F/m is a vector pointing toward the mass, while the field-line spacing only indicates how strong that field is.
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