English

The Planets

Primary schoolInterdisciplinarySpherical planetary gravity using rounded JPL mean radii, masses and signed sidereal rotation periods

Choose a planet, then drag the orange mass probe away from it. Compare weight at the surface and one radius above it.

3D

Drag the apparatus to adjust it. Drag the background to rotate. Scroll to zoom.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Inspect eight destinations and move the same mass probe above each planet. Compare radius, gravity, weight and escape speed while keeping mass distinct from the force of gravity.

Learning goals

  • Distinguish a probe mass in kilograms from its weight in newtons.
  • Test the inverse-square variation of gravity with distance from the centre.
  • Compare planets using consistent mean-radius spherical calculations.

How to use it

  1. 1Record the weight of a 50 kg probe at the mean radius of Earth.
  2. 2Visit Mars and Jupiter without changing the probe mass.
  3. 3Double distance from the centre and test whether acceleration becomes one quarter.
  4. 4Spin Venus and compare its rotation direction with Earth.

The model

The probe mass remains unchanged when its destination changes.
Escape speed assumes no atmosphere and no further propulsion.
A height of one planetary radius doubles the distance from the centre.

Assumptions

  • Rounded physical data are from JPL planetary physical parameters; gravitational constant is 6.67430 times ten to the minus eleven in SI units.
  • Each planet is a sphere at its mean radius. Oblateness, centrifugal acceleration, atmosphere and axial tilt are omitted.
  • All planets fill the same inspection frame, so the scene is not a size comparison. Gas giants have no solid landing surface at the model radius. Surface bands and rings are schematic.

Try this in class

  1. Q1Why does a more massive planet not always have greater surface gravity?
  2. Q2What happens to weight if the probe mass doubles while altitude stays fixed?

Frequently asked questions

Why can the gravity differ from a published equatorial value?

This model uses a spherical mean radius and excludes rotational acceleration. Published effective equatorial gravity may use an equatorial radius and include rotation.

What does a negative rotation period mean?

It denotes retrograde sidereal rotation relative to the usual prograde convention. The signed period controls the spin direction, not a negative duration.