// Program to find the time spent falling through a hole in the earth assuming
// a non-uniform earth. The density of the earth is modeled using a linear
// range of densities starting from 2.2 g/cm^3 at the surface and going to
// 13.0 g/cm^3 at the core. There is a small "fudge-factor" to make this
// integrated density equal the known mass of the earth.
// This program is written in Frink:
// http://futureboy.us/frinkdocs/
//
// Alan Eliasen, eliasen@mindspring.com
earthdensity := earthmass / (4/3 pi earthradius^3)
// This finds the mass that's still below you at a given distance from the
// earth's center.
mass[dist is length] := dist^3 21.4938 1.0748347372 ((2.2-13.0) dist/earthradius + 13.0) g/cm^3 / 2.2
// Find the acceleration at a given distance from the core.
a[dist is length] := G mass[dist]/dist^2
println[mass[earthradius]]
var v is velocity = 0 m/s // Velocity at end of timestep
var stepsize is time = 1/100 s
var d is length = earthradius
var t is time = 0 s
var a is acceleration = 0 gravity
while (d > 0 m)
{
t = t + stepsize
a = a[d]
v = v + a stepsize
d = d - v stepsize
// Print results every second
if (t mod sec == 0 s)
println[(t -> sec) + "\t" + (d->km) + "\t" + (v->mph) + "\t" + (a->m/s^2)]
}
// Print final times
println[(1. t -> ["min", "sec"]) + "\t, " + (v->"mph")]