def get_gravity_force(a, b, g):
    """Gravity is a force of attraction between objects. 
    The attraction between two objects is equal to their masses
    multipled together, divided by the square of the distance between them, 
    times a constant g. This is the real-life definition of gravity. 
    """
    a_to_b = b.position - a.position
    distance = a_to_b.mag()
    force = a_to_b.scale(g * a.mass * b.mass / (distance * distance))
    return force
    a.acceleration += force
    b.acceleration -= force