We know that friction is of two types – static and kinetic. Static friction acts when there is no relative motion between the surfaces in contact. Kinetic friction takes place when surfaces rub against each other. I was wondering whether the work done by the kinetic friction can be positive, negative or zero.
-
Positive work – When an object is placed on a rapidly moving belt, it moves along with the belt but with slipping (relative motion between the surfaces exist) when there is no enough friction to prevent slipping. Here the work done by the kinetic friction is positive, as the direction of frictional force and the displacement is same.
-
Negative work – Work done by kinetic friction, when an object moving on a rough surface slows down, is negative as the direction of friction and displacement are opposite to each other.
I'm unable to think of any circumstances when the work done by kinetic friction is zero because of the following reasons:
-
Work done on an object is zero if displacement is zero. In our case, if displacement is zero, the frictional force acting on the object is static and not kinetic in nature.
-
Work done is also zero when the force and displacement are perpendicular to each other. The only example I am aware of is circular motion. As the point at which the wheel touches the ground is at rest. The nature of friction is again static.
So, can the work by kinetic friction on an object be zero?
Please note: I read the answers for the following two related questions. There is no clear explanation on the two aspects of friction (static and kinetic) in those answers. Simply they don't have enough details.
Best Answer
Hold a piece of wood against a sanding belt. In your frame, the block is not moving, but
kinetic friction is exerting a force: you have to hold the block still
energy is transferred: the block gets hot, and pieces are pulled off it