Chapter 19 · free playable model
Coulomb Courier
Aim, charge, fly. Drag the probe to throw it, dial the charge it carries, and let F = kQq/r² steer it onto the pad — slowly enough to dock.
Probe charge q
+0.00μC
- |ΣF| on probe
- 0.000N
- Speed · dock ≤ 0.60
- 0.00m/s
- Clock
- 0.0s
Session score0
- Simple definition
- Coulomb’s law gives the force between two point charges: it grows with the product of the charges and falls with the square of their separation.
- Example
- Doubling one charge doubles the force; doubling the separation cuts the force to a quarter.
F = k Q q / r²nearest source +3.0 μC at r = 1.020 m, so its term is +0.000 N
Go negative and let the +3 μC anchor pull you onto the pad.
How the physics works
F = k Q q / r² with k = 8.99 × 10⁹ N·m²/C². The force lies along the line joining the two charges: away from the source when Qq is positive, toward it when Qq is negative. Flipping the sign of q turns a wall into a tow rope.
F⃗ = Σᵢ F⃗ᵢ — with several fixed sources the terms add as vectors. The red arrow on the probe is that sum, evaluated every frame; the faint arrows are the field the sources would exert on a positive test charge.
r₁/r₂ = √(Q₁/Q₂) locates the null point between two like charges. On delivery 4 the pad sits exactly there: 4 μC and 1 μC give a ratio of 2, so the null is twice as far from the strong pillar as from the weak one.
m a⃗ = F⃗ − b v⃗ closes the loop. The probe has m = 1.00 kg and moves through a gel with linear drag b = 0.70 N·s/m, integrated by semi-implicit Euler at 240 steps per second. Cut the charge and the drag alone brings you down to docking speed — that coast is the whole skill of the game. The dashed ghost line before launch runs this exact integrator ahead of time, so what you see is what you get.