#!/usr/bin/env python3
# ............................... class particle
class particle:
  color='red'
  def __init__(self,mass,x,vx):
    self.m=mass
    self.x=x
    self.vx=vx
    self.px=mass*vx
    # ............................. move particle
  def move(self):
    self.x+=self.vx
# ................................................
pt1=particle(10,10,3.5)
pt2=particle(20,15,-2.1)
center=(pt1.m*pt1.x+pt2.m*pt2.x)/(pt1.m+pt2.m)
print("momentum1 = {:.1f}".format(pt1.px))
print("momentum2 = {:.1f}".format(pt2.px))
print("center of mass = {:.3f}".format(center))
print('color1:',pt1.color,'  color2:',pt2.color)