[iacopo.russo (OP)]

Christophe wrote to us from Switzerland to ask:

I have started cross country skiing this year, and I am struggling to understand what is going on in the classic style (diagonal stride). I have tried watching many videos, like this one:

All your weight is on only one ski nearly 100% of the time. The skis are cambered, so that when we glide, the grip zone in the middle does not touch the ground. Instructors say you have to push down on your ski so that the grip zone touches the ground when you need to kick (this misleading word is the usual one). But since your weight is already on the weighted ski, how can you weigh even more? I have tried on my scales and it works if I briskly stand on my toes, I become about 15% heavier. But when I watch the footage, I can see this is not what they do in diagonal stride. I have tried to reproduce diagonal stride standing on one foot on my scales and kicking, but there is no clear pattern emerging. So I am a little bit confused about what happens.

What do you think?

[alancalverd]

You actually glide on the weighted ski which is in full contact - or should be if it is properly calibrated. As you unweight the rearward ski it sticks and provides the friction you need to drive forward.
The physics of crosscountry skiing is very complicated and not at all well understood, but it's the second most fun you can have with your clothes on.