Gravitational field and satellite orbits
PhysicsGravitation & AstronomyAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Gravitation & Astronomy
Change the orbital radius of a satellite around Earth to see how the gravitational field strength g = GM/r², orbital speed and period change. The simulation points out the radius that gives exactly a 24-hour period — the geostationary orbit that telecom satellites need to stay fixed above one point on the ground. A 3D view lets you rotate Earth and the satellite's orbit in space, with the orbital plane, the velocity vector and the gravitational acceleration vector, using the same data as the 2D view. The Field map screen shows the superposed field, field lines and equipotentials of two or three bodies, and another screen plots g inside and outside a uniform Earth.
Lesson: Gravitational field
What it shows
A satellite in a circular orbit is in free fall: gravity alone provides the centripetal force, so g = GM/r², v = √(GM/r) and T = 2π√(r³/GM), which is Kepler's third law. At about 6.6 Earth radii the period equals one sidereal day: the geostationary orbit. The Field map screen adds the fields of two or three bodies as vectors and their potentials V = −ΣGM/r as plain numbers, with field lines and equipotentials. The last screen compares g inside a uniform Earth, g = g₀r/R, with g = g₀R²/r² outside it.
How to use
On Satellite orbits, press ISS altitude, then increase the Orbital radius and press Find geostationary orbit; switch to 3D to see the vectors. On Field map, press Earth–Moon and drag P along the line between them to find where the pulls cancel, pressing Record as you go. On g inside and outside Earth, move Distance from Earth's centre r from the centre out to 4 R⊕ and compare the two parts of the graph.
Parameters you can change
- Orbital radius 1.05–10 R⊕
- Show period calculation steps
- Screen Satellite orbits, Field map, g inside and outside Earth
- Number of bodies 2, 3
- Mass M₁ 0.01–10 M⊕
- Mass M₂ 0.01–10 M⊕
- Mass M₃ 0.01–10 M⊕
- Field lines
- Equipotentials
- g arrows on a grid
- Distance from Earth's centre r 0–4 R⊕
Questions to explore
- At two Earth radii from Earth's centre, what fraction of the surface value is g?
- Why does a satellite in a higher orbit move more slowly yet take much longer to complete one orbit?
- Why do astronauts on the ISS feel weightless even though g there is nearly 90% of its surface value?
