work done

Work done against friction refers to the energy used by an external agent to overcome friction. Usually, assuming friction is a constant, say f. Then the work done against friction is given by fs, where s is the distance travelled by the object.


Work done by friction refers to the energy dissipated by friction during the motion. As friction acts in the direction opposite to the relative direction of the two objects, there is always energy dissipation during the motion if friction exists.

For their differences, for the motion to carry out, as work is done by friction, there is some energy lost in the system to the surrounding. And so an external agent has to do work against friction (It is to input energy in the system) so that the kinetic energy of the object can be resumed.