title: Error on python
body: <p>Hi I want to plot a rocket trajectory with a loope using Eulers method. My code is(you kan skip this, the error is below it):</p>
  
  <pre><code>mo = 1000
  m = 900
  q = 2.8
  u = 3700
  vo = 0
  xo = 0.0001
  g = 9.81
  t = np.arange(0,int(1001))
  x0 = 0.0001
  v0 = 0
  N = 1000
  at = 1/1000
  </code></pre>
  
  <p>Now velocity and height ecuations:</p>
  
  <pre><code>def v(t):
      return  u*np.log(mo/(mo-q*t))-g*t
  
  def az(t):
      return xo + vo*t - 0.5*g*t**2 + u*t*np.log(mo) + (u/q)*((mo - q*t)*np.log(mo 
      - q*t) + q*t - mo*np.log(mo))
  
  z = xo + vo*t - 0.5*g*t**2 + u*t*np.log(mo) + (u/q)*((mo - q*t)*np.log(mo 
  - q*t) + q*t - mo*np.log(mo))
  </code></pre>
  
  <p>And here is the aceleration wich I turned into a list because of the error:</p>
  
  <pre><code>aj = -g + (q/(mo-q*t))*u
  ajj = np.array(aj).tolist()
  </code></pre>
  
  <p>So here is the loop where I am trying to solve it in an analitic way with eulers method:</p>
  
  <pre><code>t = np.zeros(N + 1)
  t[0] = 0
  z = np.zeros(N + 1)
  z[0] = x0
  
  v = np.zeros(N + 1)
  
  v[0] = 0
  
  
  for k in range(0,N):
      v[k + 1] = v[k] +ajj*z[k]*at
      z[k + 1] = z[k] + v[k + 1]*at
  </code></pre>
  
  <p>And it gives me this error:</p>
  
  <pre><code>ValueError                                Traceback (most recent call last)
  &lt;ipython-input-193-d1513ed52aa0&gt; in &lt;module&gt;()
   11 
   12 for k in range(0,N):
   ---&gt; 13     v[k + 1] = v[k] + ajj*z[k]*at
   14     z[k + 1] = z[k] + v[k + 1]*at
   15 
  
   ValueError: setting an array element with a sequence.
  </code></pre>
  
  <p>I know what it barely means. The error is because of the list ajj but I dont know how to fix it in order to plot z along the time. Thanks for helping. Sorry for the long question I`m just starting programming.</p>