"Work done by friction" refers to the energy that is lost when two surfaces are in contact and moving relative to each other. This energy is converted into heat and sound, and ultimately results in a decrease in the velocity of the objects.
Work done by friction can be calculated by multiplying the force of friction by the distance over which the objects move. This is represented by the equation W = Fd, where W is work, F is force, and d is distance.
The amount of work done by friction is affected by the type of surfaces in contact, the force pressing the surfaces together, and the speed at which the surfaces are moving. Rougher surfaces and higher forces will result in more work done by friction.
In most cases, work done by friction is negative because it represents the loss of energy. However, if the direction of motion is opposite to the direction of the force of friction, then the work done by friction will be positive.
Work done by friction can be reduced by using smoother surfaces, reducing the force pressing the surfaces together, and minimizing the speed at which the surfaces are moving. Lubricants can also be used to decrease the amount of friction between surfaces.