関連ワード:  work done by frictional force is negative or positive    work done by frictional force is negative positive or zero    work done by frictional force is always negative or positive    work done by frictional force may be positive negative or zero    work done against friction force is positive or negative    is work done by frictional force always negative    what is the work done by frictional force  
結果 : work done by frictional force is negative or positive