Merging two columns into one

Suppose I have

file1.txt

1 2 
4 5 
10 11

and I want to produce

1 
2 
4 
5 
10
11

file2.txt

Thanks for your help

$ for i in $(cat file1.txt); do echo $i; done
1
2
4
5
10
11

---------- Post updated at 09:29 PM ---------- Previous update was at 09:28 PM ----------

$ tr ' ' '\n' < file1.txt
1
2
4
5
10
11

---------- Post updated at 09:30 PM ---------- Previous update was at 09:29 PM ----------

$ sed 's/ /\n/g' file1.txt
1
2
4
5
10
11

---------- Post updated at 09:30 PM ---------- Previous update was at 09:30 PM ----------

$ perl -pe 's/ /\n/g' file1.txt 
1
2
4
5
10
11

---------- Post updated at 09:31 PM ---------- Previous update was at 09:30 PM ----------

$ awk '{for(i=1;i<=NF;i++){print $i}}' file1.txt 
1
2
4
5
10
11

thanks alot!