Physics A2 · Gravitational fields. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The field strength around a point mass M comes from two equations. Newton's law gives the force on a small mass m as F = GMm/r2. The definition of field strength is g = F/m. Dividing, m cancels, which leaves g = GM/r2. So g depends only on the mass causing the field and the distance from its centre.
This is an inverse square law: at twice the distance from the centre, g is one quarter as large. The equation also works at and above the surface of a uniform spherical planet.
Near the Earth's surface, a change in height of a few kilometres is tiny compared with the Earth's radius. So r hardly changes and g is approximately constant.
When g falls to one quarter, r has doubled to 2R, but the height above the surface is only R. Students often give 2R as the height. Height = r − R.
Answer: About 8.9 N kg−1 (8.95).
Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.
← Gravitational force between point massesGravitational potential →Take a Physics A2 test