title: RuntimeWarning: invalid value encountered in divide
body: <p>I have to make a program using Euler's method for the "ball in a spring" model</p>
  
  <pre><code>from pylab import*
  from math import*
  m=0.1
  Lo=1
  tt=30
  k=200
  t=20
  g=9.81
  dt=0.01
  n=int((ceil(t/dt)))
  km=k/m
  r0=[-5,5*sqrt(3)]
  v0=[-5,5*sqrt(3)]
  a=zeros((n,2))
  r=zeros((n,2))
  v=zeros((n,2))
  t=zeros((n,2))
  r[1,:]=r0
  v[1,:]=v0
  for i in range(n-1):
      rr=dot(r[i,:],r[i,:])**0.5
      a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
      v[i+1,:]=v[i,:]+a*dt
      r[i+1,:]=r[i,:]+v[i+1,:]*dt
      t[i+1]=t[i]+dt
  
      #print norm(r[i,:])
  
  plot(r[:,0],r[:,1])
  xlim(-100,100)
  ylim(-100,100)
  xlabel('x [m]')
  ylabel('y [m]')
  
  show()
  </code></pre>
  
  <p>I keep getting this error:</p>
  
  <pre><code>a=-g+km*cos(tt)*(rr-L0)*r[i,:]/rr
  RuntimeWarning: invalid value encountered in divide
  </code></pre>
  
  <p>I can't figure it out, what is wrong with the code?</p>