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According to the Wiki Friction entry, Platinum on Platinum (dry and clean) has $\mu_s=1.2$ and $\mu_k=3$, which means that $\mu_s<\mu_k$. This seems impossible to me. The same phenomenon occurs in other materials, like Aluminium-Aluminium and Silver-Silver. Is this due to a large experimental error, or could this be a reality?

Why this seems strange: Lets take a Platinum box weighing $1N$ on a Platinum table. The maximal static friction would be $1.2N$. What happens when we push with $F=1.2N$ and slowly increase the force? By definition it should start moving (speeding up), but $f_k=3N>F$, so instead of accelerating, we are slowing down- from an initial velocity of $0$!

Rd Basha
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1 Answers1

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If you check the references in the Wikipedia article you linked, you will see that the source for the kinetic is different from the source for the static coefficient of the metals your mention (platinum, silver and aluminum). That implies that the experimental methods and circumstances used to determine the coefficients could have been radically different for the two sets of coefficients.

Measuring these coefficients presents a large amount of uncertainty, so to compare kinetic vs static, one should use the same samples, under the same conditions, using similar methods. Also, the principle of "does this answer make sense" should strongly apply. A sliding coefficient of friction of 3.0 for metal on metal is so far outside the expected range that it points to the high probability of a transcription error in the sources.

Looking at the source (ref [26]) of the Wikipedia article, there are many broken links, and I couldn't see the table which purported to have the value.

I strongly suspect that the static coefficient values are in error.

And that is why one should not use Wikipedia as a primary source for values in important calculations. Check the references, and the references of the references, if possible. Errors being requoted as authoritative are dangerous!

Bill N
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