## Oct. 1, 2002, A. Middleton
## gravity.py
## This module defines two functions - one calculates the
## acceleration due to gravity on a body at position ra due
## to an object at position rb with mass mb. The second function
## updates the position, velocity, and track of a planet. The
## planet must be initialized properly (see fakesolarsystem.py)
## and have a list of "others": these are the planets which exert
## a force on it.

from visual import *

forcetype = -3

dt = 0.01

## Acceleration in the direction separating objects a and b, with magnitude
## 1/distance**2. ('one over r squared' law.)
def gravaccel(ra, rb, mb):
    return(-(ra-rb)*mag(ra-rb)**forcetype*mb)

## Update position based on velocity. Velocity changes by acceleration,
## with acceleration given by the sum from all 'others'. Also update track.
def update(planet):
    planet.pos = planet.pos + dt * planet.vel
    totalaccel = vector(0,0,0)
    for other in planet.others:
        totalaccel = totalaccel + gravaccel(planet.pos, other.pos, other.mass)
    planet.vel = planet.vel + dt * totalaccel
    planet.track.append(pos=planet.pos)