penProjectionOntoDeskNormal.set(deskSurfaceNormal);
penProjectionOntoDeskNormal.scale(penToDeskDistance.getDoubleValue());
penProjectionOntoDesk.set(penPosition);
penProjectionOntoDesk.sub(penProjectionOntoDeskNormal);
penProjectionInDeskFrame.set(penProjectionOntoDesk);
transformFromWorldToDesk.transform(penProjectionInDeskFrame);
// Only apply a force on the pen, not the desk. Doesn't follow Newton's third law, but the force is probably pretty small anyway.