関連ワード:  is work done by frictional force always negative    work done by frictional force is always negative true or false    work done by frictional force is always negative or positive    is the work done by kinetic friction force always negative    work done by frictional force is positive or negative or zero    work done by frictional force is positive or negative    is work done by friction always negative    is work done by friction negative    is frictional force always negative  
結果 : is work done by frictional force always negative