# Friction at high speeds

I am reading a book which mentions that frictional force is not exactly proportional to the normal force at high speeds. Quoting:

Once the bodies slip on each other frictional force is, $$kN$$. This is approximately true for relative speeds not too large (say for speeds <10 m/s).

$$k = Frictional constant$$ $$N = Normal force$$

If so is the case,then what is the general way to find out the frictional force?

• The units of speed? m/s or km/hr ... – user207455 Jul 12 '19 at 13:48
• Related, though not an exact duplicate: Is friction always constant?. – John Rennie Jul 12 '19 at 14:10
• @SolarMike m/s, sorry I missed that – user8850564 Jul 12 '19 at 15:29
• @JohnRennie Although the answers in that question touch the topic, they don't clear it enough. BTW thanks for linking that question. some good stuff there. – user8850564 Jul 12 '19 at 15:34
• @user8850564 yes, which is why I didn't flag it as a duplicate. I don't think there is a simple answer to your question as the interaction between the surfaces is a complicated one. – John Rennie Jul 12 '19 at 15:42