void PopulationTestingForce<ELEMENT_DIM, SPACE_DIM>::AddForceContribution(AbstractCellPopulation<ELEMENT_DIM,SPACE_DIM>& rCellPopulation)
{
    for (unsigned i=0; i<rCellPopulation.GetNumNodes(); i++)
    {
        c_vector<double, SPACE_DIM> force;

        for (unsigned j=0; j<SPACE_DIM; j++)
        {
            if (mWithPositionDependence)
            {
              force[j] = (j+1)*i*0.01*rCellPopulation.GetNode(i)->rGetLocation()[j];
            }
            else
            {
                force[j] = (j+1)*i*0.01;
            }
        }
        rCellPopulation.GetNode(i)->ClearAppliedForce();
        rCellPopulation.GetNode(i)->AddAppliedForceContribution(force);
    }
}