# What exactly causes skin rubbing against skin to have so much friction? **URL:** \ **Category:** Factual Questions\ **Created:** [June 1, 2018, 6:30pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274 "2018-06-01T18:30:43Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Contemplation](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@Contemplation](https://boards.straightdope.com/u/Contemplation)\ **Post date:** [June 1, 2018, 6:30pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/1 "2018-06-01T18:30:43Z")
Everything has friction, but why is it that when skin is rubbing on skin the friction is so great? Think of what happens to a person’s inner thigs when running a long distance. There’s a lot of differant theories about this. Some people say it’s the salt in your sweat that increases the friction between skin rubbing. But is it really the salt, or is just the water by itself? Some people say that when too smooth surfaces come in contact, they tend to adhere to each other. On the other hand… Some say that rough surfaces should have more friction because the bumps in the surface are interlocking. Is skin rough or smooth? So which one of these things is the cause for all that friction? ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [June 1, 2018, 6:51pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/2 "2018-06-01T18:51:18Z")
Are you sure it is all because of friction and not just the softening effect that moisture has on our skin. Prolonged wet skin has no toughness and doesn’t stand up very well to even small amounts of friction. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [June 1, 2018, 6:58pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/3 "2018-06-01T18:58:25Z")
Well, wetting the skin certainly _does_ increase friction. That’s why you spit in your palms before grabbing the shovel or the oars, right? I guess it has something to do with creating more hydrogen bonding between the surfaces. As for the inner thighs, my guess is that they would be a lot less afflicted by frictional wear if we ran naked. You know, the old trick to unscrew the lid from a jar is to stick a piece of damp cloth between it and your hand! ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [June 1, 2018, 8:09pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/4 "2018-06-01T20:09:02Z")
sweat increases the friction ---
**Author:** ![Tatterdemalion](https://avatars.discourse-cdn.com/v4/letter/t/43a26b/32.png) [@Tatterdemalion](https://boards.straightdope.com/u/Tatterdemalion)\ **Post date:** [June 1, 2018, 8:32pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/5 "2018-06-01T20:32:10Z")
There was a girl I knew in college who mentioned that she was really enjoying freshman physics. When asked what she liked so much about it, she gave a little smile and replied, “friction!” ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [June 1, 2018, 8:45pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/6 "2018-06-01T20:45:29Z")
> [@Contemplation](#): > > Everything has friction, but why is it that when skin is rubbing on skin the friction is so great? Think of what happens to a person’s inner thigs when running a long distance. I guess this example is for overweight people? I can’t make my inner thighs touch unless I cross my legs. > [@Contemplation](#): > > Some people say that when too smooth surfaces come in contact, they tend to adhere to each other. Friction is quite complicated and can be nonintuitive (for example, it’s not intuitively obvious that the frictional force is, to a first approximation, independent of the surface area of the objects in contact). But generally rougher surfaces have more friction. What you may be referring to is the intermolecular attraction that occurs when two EXTREMELY smooth surfaces like Johansson blocks are brought into contact. This requires rigid objects machined to a microscopically perfect smoothness, and would not apply to something like human skin. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [June 1, 2018, 10:47pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/7 "2018-06-01T22:47:21Z")
> [@Ignotus](#): > > Well, wetting the skin certainly _does_ increase friction. That’s why you spit in your palms before grabbing the shovel or the oars, right? Isn’t the point of doing that to _reduce_ friction in order to prevent blisters?? ---
**Author:** ![Sicks\_Ate](https://avatars.discourse-cdn.com/v4/letter/s/258eb7/32.png) [@Sicks\_Ate](https://boards.straightdope.com/u/Sicks_Ate)\ **Post date:** [June 2, 2018, 12:12am UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/8 "2018-06-02T00:12:28Z")
> [@Bear\_Nenno](#): > > Isn’t the point of doing that to _reduce_ friction in order to prevent blisters?? Depends on what you’re spitting on, I think. ---
**Author:** ![Beckdawrek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beckdawrek/32/3647_2.png) [@Beckdawrek](https://boards.straightdope.com/u/Beckdawrek)\ **Post date:** [June 2, 2018, 4:59am UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/9 "2018-06-02T04:59:34Z")
> [@Omar\_Little](#): > > sweat increases the friction Reason #127 for not excercising. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [June 2, 2018, 6:14am UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/10 "2018-06-02T06:14:11Z")
> [@Ignotus](#): > > You know, the old trick to unscrew the lid from a jar is to stick a piece of damp cloth between it and your hand! Is this a joke (hence the exclamation mark)? ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [June 2, 2018, 12:19pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/11 "2018-06-02T12:19:36Z")
The answer (per actual studies more than what some people say) is in fact [complicated](http://www.ippt.pan.pl/Repository/o3643.pdf) and more than just rough or smooth. Skin has a complex geometry at the microstructure level and it deforms. And moisture, such as from sweat or the environment, decreases the stiffness of the upper layer by orders of magnitude, allowing for more deformation and more contact area. From that article skin _elasticity_ seems to be a larger factor in skin friction than skin roughness/smoothness. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [June 2, 2018, 12:50pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/12 "2018-06-02T12:50:27Z")
> [@Contemplation](#): > > Think of what happens to a person’s inner thigs when running a long distance. Wouldn’t this be because of the volume of friction? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [June 2, 2018, 10:35pm UTC](https://boards.straightdope.com/t/what-exactly-causes-skin-rubbing-against-skin-to-have-so-much-friction/815274/13 "2018-06-02T22:35:45Z")
> [@Contemplation](#): > > Everything has friction, but why is it that when skin is rubbing on skin the friction is so great? Think of what happens to a person’s inner thigs when running a long distance. It’s not particularly great, or your final sentence here would have ended “when a person takes a short walk”. Sent from my XT1710-02 using Tapatalk