Are kinetic frictional forces always slowing down objects?

[keltix]

Do kinetic frictional forces always slow objects?

 

[mordechai9]

Yes. By definition, kinetic frictional forces are resistive forces, and they act in a direction opposite of the motion of that object. As an example, you might think about a smooth block sitting on a steep, snow covered hill. You might imagine that the block will slide down the hill, and therefore, you might get confused, thinking that the frictional force is actually speeding up the object. In fact, it is the weight of the object which is causing it to speed up, and the frictional force indeed opposes this motion, albeit, in this example, perhaps with negligible strength.

Now, this answer corresponds to the classical concept/definition of friction. If you're talking about electromagnetic interaction or something like that when you say the word "friction", then that's a different matter.

 

[Doc Al]

Do kinetic frictional forces always slow objects?

Generally, yes, but not always. Imagine a box on the floor of a railroad flatcar which is accelerating faster than static friction can support. In that case, kinetic friction between the floor and the box will accelerate the box (until it slides off or bangs into a wall).

But kinetic friction always dissipates mechanical energy (transforming it into thermal energy). And it always acts to oppose slipping between surfaces. In the case of the box and the train, the box tends to slip backwards (with respect to the train) thus friction acts forwards. But viewed from the tracks, the friction force on the box points in the same direction as the box's motion.