projectStar Trek Lab
nameDylan DuanGrade 11 Physics
problemDropping a red-matter probe down a hole to the centre of Vulcan
givenR = 6.79 × 10⁶ m, g₀ = 13.7 m/s², population 7.00 × 10⁹

Aim

How long does the probe take to fall from 2.00 × 10³ km above the surface, starting from rest, to the centre of Vulcan, if Vulcan is the same density all the way through and we ignore air? And how many Vulcans per second is that?

(you can drag the dotted numbers and click the wavy words to change them)

s

t = 1.36 × 10³ sso5.15 × 10⁶ Vulcans/s✓ same as the formula

Set-up

released here+2 000−2 000−4 000−6 0000 surface−6 792centreheight, km

(space bar = drop, arrow keys = step through it)

Time
0.00 s
Height
2.00 × 10³ km
Speed
0.00 m/s
Acceleration
8.19 m/s²

The probe is above the surface.

What I can change

Results

EulerExactdiff.
time to surface, t₁671 s671 s-0.000601 %
surface to centre, t₂687 s687 s0.00120 %
total time1.36 × 10³ s1.36 × 10³ s0.000312 %
speed at surface6.51 × 10³ m/s6.51 × 10³ m/s-0.000850 %
speed at centre1.16 × 10⁴ m/s——
rate (Vulcans/s)5.15 × 10⁶5.15 × 10⁶-0.000312 %

Graphs

Toward the centre is positive. The shaded part is when it's inside the planet. Hover over a graph to see where the probe was at that time, or click to move it there.

velocity vs time

Top speed is right at the centre, since g is 0 there and it stops speeding up.

acceleration vs time

Biggest at the surface, then goes down to 0 at the centre.

displacement vs time
g vs distance from centre

Outside the planet both models give the same GM/r² curve. They only split up inside.

rate vs drop height

Uniform vs dense core

Same drop, both models. Drag is on its own graph because it makes the fall take way longer, and the other lines got squished when they were together.

velocity, no drag
velocity, with drag

Working

gravity outside
g = GM/r², with GM = g₀R² = 6.33 × 10¹⁴ m³/s²
gravity inside
uniform: g = g₀r/R; dense core: g = g₀
Euler step
a = a(r), then v += a·dt, then r −= v·dt. Repeat until r = 0.
exact, outside
t₁ = √(r₀³/2GM)·[√(x(1−x)) + cos⁻¹√x], x = R/r₀
exact, inside
uniform: t₂ = tan⁻¹(ωR/vs)/ω, ω = √(g₀/R)
dense core: t₂ = (−vs + √(vs² + 2g₀R))/g₀
rate
population ÷ total fall time

All the physics code is in src/lib/physics.ts.