title: list assignment index out of range when I want to define an array
body: <p>that works and when I add this part:</p>
  
  <pre><code>print('x-coord',x_coord[0])
  
  force_line = []
  force_line[0] = x_coord[0]
  force_line[1] = y_coord[0]
  force_line[2] = 0
  </code></pre>
  
  <p>I have an error:</p>
  
  <pre><code>x-coord -0.5634698919774295
  Traceback (most recent call last):
    File "centre_motion.py", line 145, in &lt;module&gt;
      force_line[0] = x_coord[0]
  IndexError: list assignment index out of range
  </code></pre>
  
  <p>What is wrong, please?</p>