Floating point calculation problem

[Schizoid Man]

I have a program that performs some calculations that runs perfectly on 
Python 2.7.3. However, when I try to execute it on Python 3.3.0 I get the 
following error:
    numer = math.log(s)
TypeError: a float is required

The quantity s is input with the following line: s = input("Enter s:   ")

To get rid of the compile error, I can cast this as a float: s = 
float(input("Enter s:   "))

However, then the result returned by the method is wrong. Why does this 
error occur in version 3.3.0 but not in 2.7.3? Why is the result incorrect 
when s is cast as a float (the casting is not required in 2.7.3)? How is 
Python dynamically typed if I need to cast (in version 3.3.0 at least) to 
get rid of the compile error?

Thanks in advance.

PS - I'm a Python newbie.