_getForceContributions(parentBranch, node) {
    this._getForceContribution(parentBranch.children.NW, node);
    this._getForceContribution(parentBranch.children.NE, node);
    this._getForceContribution(parentBranch.children.SW, node);
    this._getForceContribution(parentBranch.children.SE, node);
  }
@param {object} parentBranch
@param {Node} node
@private