Cannot Compile A Program With Jar File

I created Myproject folder. Inside I have 3 folders:

/lib
/src
/bin

Inside src there is a .java file:

public class hello_world{
public static void main(String[] args){
System.out.println("Hi, from hello_world");
seba.st.hello_world_package test1 = new seba.st.hello_world_package();
test1.packFunc();
}
}

inside lib is a packEx.jar file which I created from this .java file:

package seba.st;
 public class hello_world_package{
  public void packFunc(){ 
System.out.println("hi from pack_func!");
}
}

I am trying to run this program from terminal with this command

javac -d bin -sourcepath src -cp lib/packEx.jar src/hello_world.java

and I get this error:

src/hello_world.java:11: error: cannot find symbol
test1.packFunc();
^
symbol: method packFunc()
location: variable test1 of type hello_world_package
1 error

What am I doing wrong ? How can I compile and run this program from terminal?