your link says …
The force of friction does not do work upon the cart because it acts upon the wheels of the cart and actually does not serve to displace either the cart nor the wheels. … i've read something like this many times, but it's wrong!
The friction force only serves to help the wheels turn as the cart rolls down the hill.
Friction only does work upon a skidding wheel.
friction (from the road) does do work on a rolling wheel …
in the obvious case of an accelerating car on a horizontal road, the only external
horizontal force is the friction (from the road) …
if it's not doing the work, what is??!! work done
= force "dot" displacement
of the point of application of the force, the point of application is the point of contact with the road, and that's moving!
suppose the car has mass M, and the wheels have total mass m radius r and total moment of inertia
if a horizontal force P pushes the car (on a horizontal road), and if the total friction force is F, and if the reaction force
between the car and the wheels is R, then …
P - F = (M+m)a
Fr = Ia/r
so P - (I/r2)a = (M+m)a,
or P = (M+m+mr)a, where mr is the "rolling mass" I/r2
in other words: either
we must take F (the friction) into account, or
we must add the "rolling mass" to the actual
mass of the wheels and the car