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
(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.
Results
Euler
Exact
diff.
time to surface, t₁
671 s
671 s
-0.000601 %
surface to centre, t₂
687 s
687 s
0.00120 %
total time
1.36 × 10³ s
1.36 × 10³ s
0.000312 %
speed at surface
6.51 × 10³ m/s
6.51 × 10³ m/s
-0.000850 %
speed at centre
1.16 × 10⁴ m/s
—
—
rate (Vulcans/s)
5.15 × 10⁶
5.15 × 10⁶
-0.000312 %
dt = 0.01 s, all answers to 3 s.f.
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
UniformDense core
Outside the planet both models give the same GM/r² curve. They only split up inside.
rate vs drop height
uniform, dt = 0.05 s
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
UniformDense core
velocity, with drag
UniformDense core
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.