If we have a box with mass m placed on a horizontal tabletop. Then we push the box a distance of d. THe coefficient of kinetic friction between the box and tabletop is .35. When we're asked to find the external work done on the block-table system, it's not the work done by the friction force at all. Why??? Can someone explain to me?? My textbook refers the work done by friction is transfered to thermal energy. So they don't call it the external work. But in the previous before, they say friction force is external force.