MOVE THE SENSOR. FEEL THE FIELD.
Which way does gravity point?
Place masses, move the sensor and watch their gravitational fields add as vectors.
Explore the field
Sensor readyPan view: drag anywhere. In Move objects mode, drag the sensor or a mass; drag empty space, right-drag, or hold Space and drag to pan. Use the buttons or mouse wheel to zoom out and back in. No rotation.
● Sensor → resultant fieldMass labels select the object to editDashed arrows: individual contributions
Arrow length = field strength × the chosen scale. Moving the sensor does not change the camera or arrow scale. Camera zoom changes the distance view; the separately chosen arrow scale stays unchanged. Point-mass mode magnifies its tiny fields using a clearly labelled scale. All arrows share the same scale. The sensor is a test probe and does not create a field.
How the masses combine
| Source | Distance / km | gₓ / N kg⁻¹ | gᵧ / N kg⁻¹ | Region |
|---|
Add the x components and the y components separately. The strength of the resultant is the magnitude of this vector sum, not the sum of the individual strengths.
Outside, inside and between masses
Point mass, or outside a spherical planet
The field points towards the source centre and decreases with the square of distance. An ideal point mass has an undefined field at its exact position; this simulation reports that singularity instead of inventing a finite value.
Inside a uniform-density planet
For an isolated uniform sphere the field decreases linearly towards zero at its centre. At the surface, the inside and outside expressions agree. Other masses can still produce a field at that centre.
Newtonian gravitational field strength is force per unit test mass, in N kg⁻¹ (equivalently m s⁻²). Uniform-density spheres are an idealization: real planets have layered, non-uniform interiors.
Model and display
G = 6.67430 × 10⁻¹¹ N m² kg⁻². Point masses can range from 0 to 100 kg; a zero-mass marker produces no field, including at its own location. Planet masses are positive. The two modes use separate mass and distance scales; their saved setups remain separate. Uniform spheres use the shell theorem and may be superposed mathematically, even when the drawn circles overlap; overlapping spheres represent overlapping density distributions, not realistic solid planets. Positions lie in a plane, but the gravitational law and spheres are three-dimensional. Point-mass icons are not physical radii. Very long arrows can leave the view; their lengths are not silently capped or normalized. Reduce the arrow scale when needed. The maximum is twelve sources to keep the display manageable.
Explore gravitational fields
Move a test sensor around multiple masses and observe the strength and direction of their resultant gravitational field. Use a point-mass system with 0–100 kg sources and metre-scale distances, or a separate planet system with astronomical masses, radii and distances. Add, remove and reposition sources while comparing individual field contributions.
Field arrows share a clearly labelled proportional scale. Examine inverse-square behaviour outside a sphere and the linear field inside a uniform-density planet, including zero field at its centre when isolated. Equal masses demonstrate cancellation. Pan and zoom without changing the physics or arrow scale. This interactive resource supports IB, A Level and AP Physics learning.