Car crash reaction time In physics class we are doing an investigation thing. Some of the numbers might seem plucked out of thin air but they aren't. The maths is right(I think) I just want some feedback on the code.

import datetime
import random
import time
import math


player_1 = input("Player one: ")
speed = int(input("Speed in km/h: "))
speed_ms = ((speed)/3.6)
conditions = input("Wet or dry conditions? (all lowercase)")

if conditions == "wet":
    friction_coefficient = 0.4
else:
    friction_coefficient = 0.6


time.sleep(0.5)
print("Get ready..")
time.sleep(random.randint(1,12))
then = datetime.datetime.now()
t = input("GO!! ")
now = datetime.datetime.now()
diff = then-now
reaction_time = round(abs(diff.total_seconds()), 2)


Rt = (math.sqrt((reaction_time*98)/490))
print('Your final reaction time was: ', (Rt), '!\n')

Rd = (Rt * speed_ms)
print('Your final reaction distance was: ', (Rd), '!\n')

Bd = (((speed_ms)**2) / 19.6 * (friction_coefficient))
print('Your final breaking distance was: ', (Bd), '!\n')

Sd = ((Rd)+(Bd))

############################################################################

print("\n\n---===========FINAL STOPPING DISTANCE===========---")
print('--------------', (Sd), 'meters', '------------')
Some of the numbers might seem plucked out of thin air but they aren't


Still better have them explained. I can understand 3.6 and even 19.6 (it is \$2g\$, isn't it?), but I have no idea what 98/490 stands for. Declare them as symbolic constants with meaningful names.
I don't see a need for abs in

    reaction_time = round(abs(diff.total_seconds()), 2)


Here diff may not be negative. If it is, I'd rather throw an exception, because the result is meaningless anyways.
You are rounding too early. There is no penalty computing Rt with reaction time not being rounded, but rounding leads to loss of precision. Only round when printing results.
You have too many parenthesis. Sd = Bd + Rd is much cleaner than Sd = ((Rd)+(Bd)).