In a two dimensional space the potential energy function of the only conservative force acting on a particle of mass $m=0.1$  kg is given by  $U=2(x+y)$  joule [ x & y are in metres]. The particle is being moved on a circular path  $x^2+y^2=4$  with constant speed  $V=1m/s$. The y component of the net external force [other them the conservative force] that must be acting on the particle at [0,4] in newton’s is _______