# Why do you hear an acute sound when you rub your shoe on the floor?

After raining when I get home I rub my shoe's wet outsole on the floor and I hear an extremely acute (high pitch) sound (might be important to say my shoe's outsole is made of rubber and the floor is wooden).

Why does a wet rubber on a wooden floor create such a high pitch sound? Does friction have anything to do with this?

Thanks.

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Slip and stick

The friction between your shoes and the floor is quite non-linear. As you put your foot down, your shoe comes under elastic strain as your foot moves forward but friction holds the bottom in place. This is the stick phase. As your foot moves forward, the strain becomes too big, and the friction between floor and shoe can't overcome it anymore.

The bottom of the shoe starts to move. The dynamic friction now is lower than the static friction before, which means the acceleration is rather large. However, the strain also dissipates quickly as the shoe flexes back to its original form. This slip phase is therefore also time-limited.

So, when you hear a high-pitched squeak, you're hearing your shoe stick and slip several thousand times per second, moving micrometers at a time.

Water definitely affects the static and dynamic friction, so that explains why it can matter. But note that there's no simple rule about the exact change in friction, and therefore adding water may also prevent squeaky noises in other situations (when it acts as a lubricant)

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Thanks for the answer. – Dayman75 Feb 5 '14 at 14:26
Might be worth adding that the sound is so high-pitched because the rubber is quite firm (so the strain becomes too big after only little flexing) and there's not much inert mass that needs to be moved in the slip phase. Apart from these parameters, it's basically the same process as when you use a double-bass bow to produce much lower-pitched sounds. – leftaroundabout Feb 5 '14 at 16:38
Can't edit your answer to fix the typo of "shows" to "shoes"... (apparently I have to change at least 6 characters to make an edit worthwhile...) – Michael Feb 5 '14 at 16:44
Adding to the bit about water affecting the friction. When the rubber is in stick mode the water has been pressed out of the way, while in slip mode the water is acting as a lubricant. So with water the difference between the static and dynamic friction is much higher, which makes the noise much louder and easier to achieve. – Rick Aug 14 '15 at 19:27