'''
Air bubbles "trapped" in a viscous liquid will slowly rise to the surface. The 
velocity of these bubbles can be calculated by solving the force balance 
equation: 

k * v = (rho_V - rho_w) * V * g

where k is a drag coefficient, v the velocity of the bubble, rho_V the density 
of the air bubble, rho_w the density of the liquid, V the volume of the bubble 
(assumed to be spherical), and g the acceleration due to gravity. All numbers 
should be given using standard SI units (kg, m, s, etc.). 

The outline code to calculate the air bubble velocity is given below. 

Your task is to:
* Complete the code, where the diameter is asked as input value by the user 
in cm. 
* Calculate the volume of the spherical air bubble.
Use 3.14 as the value of pi.
* Print the velocity of the air bubble and the associated unit, e.g., 
"The velocity is:  .... m/s.".
* It is considered good practice that the units are given directly after the 
numbers (as shown with g). Provide the three missing units. 
'''

k = 0.25 # Add unit
rho_V = 1.29 # Add unit
rho_w = 0.998 # Add unit
g = 9.81 # m/s^2

# Your code for defining d and V

# Your code for calculating the air bubble velocity and printing output