Newtonian Force Calc for Squash

import math
import numpy as np
print("------------------------------------------")
print("- Newtonian Force calculator for Squash  -")
print("------------------------------------------")
mass = float(input("Enter mass in kg = "))
mass - abs(mass)
initialSpeed = float(input("Inital Speed in m/s = "))
bouncebackSpeed = float(input("Negative Speed in m/s = "))
time = float(input("time = "))
Pi = mass * initialSpeed   
Pf  = mass *(bouncebackSpeed)
I = Pf - Pi
F = I/time
print(Pi,"kg. m/s Pi")
print(Pf,"kg m/s Pf")
print("= ",abs(F), " Newtons (N)")