code: function(body, jx, jy, r){
  //  body.v = body.v.add(vmult(j, body.m_inv));
      body.vx += jx * body.m_inv;
      body.vy += jy * body.m_inv;
  //  body.w += body.i_inv*vcross(r, j);
      body.w += body.i_inv*(r.x*jy - r.y*jx);
  }
docs: /*
  var apply_impulse = function(body, j, r){
  body.v = vadd(body.v, vmult(j, body.m_inv));
  body.w += body.i_inv*vcross(r, j);
  };
  
  var apply_impulses = function(a, b, r1, r2, j)
  {
  apply_impulse(a, vneg(j), r1);
  apply_impulse(b, j, r2);
  };