Sheffield Tony":2h5b2uvf said:
Belatedly picking up on this interesting question... I found the answers in a 16 page article in the TATHS Journal (Vol 14, 2005) by Warren Hewertson.
1 - Your school physics was correct - friction is independent of the surface area, but increases with force (which is often the force of gravity on a mass, ie weight). This was understood by Leonardo da Vinci but not widely known for a long time.
2 - In the absence of understanding, the salesmen move in. It seems that early metal planes were rejected as being hard to push, so their manufacturers made modifications - by cutting grooves or drilling dimples in the sole - which they claimed reduced friction. They did not. The best they may have done was to make the sole easier to flatten, or in some cases make a reduction in weight, though this was normally negligible.
It does seem remarkable that planes with grooved soles remain on sale when we all ought to know better.
I shall continue to scribble on the sole of my (plain) planes with a bit of old candle! That's a much better way to reduce friction.
[The TATHS Journal is available here through the Librarian, as are some others.]
