# Calculates drop time in a gravitational field Msolar = 1.989E30 kpc = 3.08567759E19 G = 6.67384E-11 Myr = 3.15569e13 M = 1E12 * Msolar r = 200.0 * kpc v = 0.0 tstep = 0.001*Myr t = 0.0 step = 0 while (r > 0.1*kpc): step = step + 1 t = t + tstep a = -(G*M)/r**2 v = v + a*tstep r = r + (v*tstep) + (0.5*a*tstep**2) print t/Myr, ' Myr' print step, ' steps.'