The distance between two probes doubles. The pull between them becomes…
The International Space Station orbits about 400 kilometers above the ground. News reports call it 'zero gravity' up there. Yet Earth's field at that height still has about 90% of its surface strength.
This lesson explains why g shrinks as you move away from a planet, and why it shrinks so slowly close in.
Earth's gravitational field is radial: at every point it points straight in toward Earth's center.
Draw that field and you draw field lines pointing inward. Follow the lines outward, and they spread farther and farther apart.
You've already met the reading rule for spacing: where field lines are closer together, the field is stronger.
Near Earth, the inward lines sit bunched tight. Far away, the same lines have spread over a huge sphere.
A probe coasts from Earth's surface out into deep space. What happens to g at the probe?
How fast does it weaken? You've already seen the answer for the gravitational force between two objects. The force follows an inverse-square law: doubling the distance between centers cuts the force to one quarter.
g follows the same inverse-square pattern, because g is just the gravitational force on each kilogram. Double your distance from Earth's center, and g drops to one quarter of its surface value.
Close to the surface, though, the drop is tiny. The station is only about 6% farther from Earth's center than the ground is.
The space station flies about 400 km up. Why is g there only a little smaller than at the ground?
The station is only about 6% farther from Earth's center than the ground is.
Over that small extra distance, the field's lines have barely spread.
g at the station's altitude is still about 90% of surface g: about 8.7 N/kg against 9.8 N/kg.
Viktor says Earth's field at the space station's altitude still has about 90% of its surface strength. Is he correct?
A deep-space probe triples its distance from Earth's center. What happens to g at the probe?
Tripling the distance spreads the same field lines over nine times the area.
g follows the inverse-square law, so it falls to one ninth.
g at the probe is one ninth of its earlier value.
There is no height where Earth's field switches off. The lines keep going, thinner and thinner, without end.
Esme says Earth's gravitational field ends where the atmosphere ends. Is she correct?
The Moon's field also weakens with distance from the Moon. Which statement explains why?
A probe doubles its distance from Earth's center. What happens to g at the probe?
At Earth's surface, g = 9.8 N/kg. A probe sits at twice Earth's radius from Earth's center. Enter g at the probe, in N/kg.