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The Physics of a Slap

  1. Jul 19, 2011 #1
    I'm not sure if this is the right forum, but here goes.

    I was thinking about the physics of a slap, or a "slappy" feel where an impact stings on the skin but not much deeper than that. My question is what's going on there? I'm pretty sure it has to do with the rigidity of the object doing the slapping. In fact, that's the basis for the question. And more importantly is there any way of determining whether something will feel "slappy"?

    For example, my hand or even a rubbery mousepad will feel "slappy", whereas a computer mouse itself will not. But if I wrap my hand in a towel, it won't feel "slappy" whatsoever.

    I realize this is a little silly, but I'd like to know what's going on in terms of physics, forces and momentum.
  2. jcsd
  3. Jul 19, 2011 #2


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    Grats for so kinky question! Rare at this forum!

    I don't think it is related to mass/acceleration, etc. As for my experience "slappiness" is rather related to sudden, simultaneous stimulation with pretty uniform (in a small - below 1mm scale) pressure of large part of skin. Just compare slappiness of slaps made with a rubber strap, leather belt, hand, dense fabric, frotte towel. Of course, the stimulation force must stay in some limits deep between lovely touch and jaw breaking.

    ADDED >>>
    Maybe the acoustic part of slap is also important?

    LANG >>>
    Distinction between 'slappiness' and 'spankiness' should be made by some native English speaker...
    Last edited: Jul 19, 2011
  4. Jul 19, 2011 #3
    Well, I wasn't intending it to be kinky :shy: But it just occurred to me now that the sting of a slap feel can only be felt in parts of that are somewhat fleshy, like belly, arms, legs, etc. Knuckles, knees, shoulders and head are not, and consequently don't get the same sting from a slap.

    Still, the physics of it intrigues me. Clearly flatness, or surface area, plays a role. One could get a slappy feel from a paddle, but not a baseball bat.

    The more I think about it, the more I think the physics of elastic collisions is also involved. But I'm not sure.

    One thing I am wondering about though: Is it possible to measure whether something is slappy (without a slappee being involved) and would one go about doing it?
  5. Jul 19, 2011 #4


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    You must first define what really means 'slappy' (I still hope someone would differentiate 'slappy' vs. 'spanky'...)
    It is more matter of psychology than physics. As soon psychologists define what 'spanky' means, we, physicists, may get back to the problem and compare rubber vs. frotte. But for now, the main issue is a precise definition of 'slappiness'.

    My introspection goes for a hit that is:
    1. sudden;
    2. short lasting (sub 1s);
    3. covering large area (1dm^2 or so) of skin;
    4. uniform in a sub 1mm scale (frotte is not slappy, rubber is)
    5. acousticaly 'claspy'
    6. force balanced between of friendly touch and bone breaking
    Last edited: Jul 19, 2011
  6. Jul 19, 2011 #5
    I would say the definition of slappy is where the pain of impact of an object is largely felt on the surface of a body with little to no pain (or damage) in the muscle underneath.

    Actually, I think I've figured out the why, but not so much the how. Something similar to surface tension (or maybe it IS surface tension) is being exhibited by a person's skin/fat/muscle. Any object applied to the surface of a person will have their surface get changed by the object. Like poking a finger into a flabby belly, the belly will indent around the finger. The same goes for a belt cinching into said flabby belly.

    However, if a belt were smacked into the belly quickly, the energy of the belt would be dissipated into the surface, translating into ripples and surface pain.

    That energy from the belt is different from a punch though and I'm now wondering if there's a way to generically measure that difference and if so, how.
  7. Jul 19, 2011 #6


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    It is much too weak definition!
    Take a cigarette and touch your skin. Allow a mosquito to sting you. Nurse vaccinating you against pox. That all match your definition, but those are obviously non-slappy.

    Think first about feelings (making differentiation between slap and other pains) then dynamics would be a secondary issue.

  8. Jul 19, 2011 #7
    What I said above with the proviso that the skin is not broken or visibly damaged (I'm not counting any cellular damage that may occur with any normal skin contact). Note that does not preclude bruising which DOES sometimes happen with a slap.
  9. Jul 19, 2011 #8


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    I am quite serious - the definition of 'slappiness' is a crucial issue here - and it is much-much-much more psychological and linguistic issue than physical.

    I really enjoy workining on the problem ;) But if I kick you (don't worry, not that hard, not leaving bruises) it would still fulfil your definition of 'slap', obviously not being a 'slap'.

    As English is my (poorly) learnt language, I can't make a good differentiation between 'slap' and 'spank' (both are translated to the same word in my language) nor to interprete all nuances of what 'slap' means.
  10. Jul 19, 2011 #9


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    Important components of a slap is an impact
    - over a large surface area
    - impact over that area everywhere simultaneously
    - fast
    - little mass behind it

    If a rigid object impacts you and it is flat, it will not hit over its area simultaneously. One part will sink in and do more damage. Hoever,a rigid object that's cupped so as to conform to your body (cheek, arm) will slap becaise it makes contact everywhere simultaneously.

    It needs to be fast enough to form an impact but not have much mass behind it, or it will bowl you over.
  11. Jul 19, 2011 #10


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    As far as I see, there is no difference except in the choice of target.

    A spank is a subset of slap that only applies to the butt as a target.
  12. Jul 19, 2011 #11
    I hadn't realized that English is not your first language. In English, the primary difference between slap and spank is that spank implies the buttocks is the target. The usage is generally reserved for parents chastising their children, and well... adults doing the same to other adults. Slap can be used to describe the same thing, but can also be used to describe completely different actions. Like if I said something a woman did not like, she could slap me across the face, but she would not spank me across the face (I see this much more on TV than I do in real life, BTW).

    From a physics perspective, spank and slap are virtually identical.

    A kick would leave a much deeper pain than a slap would. I think Dave's description is probably the best definition. The question is, is there a good way to measure this?
  13. Jul 19, 2011 #12
    A slap is all momentum and no force. A punch or a kick is momentum combined with force, it is not so much that there is more mass behind the contact points but that this mass is under a constant force in a punch or a kick.

    If you attempted to slap someone and kept the torque as high as possible throughout, it would not make much of a slap but just push the person over or out of the way.
  14. Jul 19, 2011 #13


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    I guess I got the difference ;) Even at physics portal you may learn some language ;)
    I just worried it might be deeper than girl is 'slapping' man's face if he is impudent to her,
    while 'spanking' is directed to butts....

    So we are close to get the definition:
    - over a large surface area (1dm^2 - maybe less - open hand - 0.5 dm^2)
    - impact over that area everywhere simultaneously (agreed - human perception clock, about 100 ms)
    - fast (sudden? not preceded by any other stimulation?)
    - little mass behind it (I understand that "and his broad aunt, the Hippopotamus, spanked him with her broad, broad hoof..." was Kipling's licentia poetica...)

    - I insist for uniformity in skin-receptors scale (0.2 mm or even less). That is what differentiates slap with leather belt from frotte-towel.

    - A slap is all momentum and no force. => force acting for a very short period of time (time < human perception limit of 0.1s) ?
  15. Jul 19, 2011 #14
    Wait, so if I slap you with a ruler because the surface area is small it is not a slap? I don't think so :biggrin:

    No force is needed at all, the force is only used to get up to speed before contact. Now if you have something with a heavy end like a hammer the momentum will be so high that it would give the same effect as having continued force behind a lighter object. So it should be light weight.

    Time for me to look into the Physics of a Nap for awhile.
  16. Jul 19, 2011 #15


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    Not really ... I don't think the amount of force behind it matters very much at all as to whether or not it is a slap. I think the earlier suggestions about the area over which the impact is distributed is more significant. No matter what, there is going to be force applied to the target as the object impacting it is rapidly decelerated. If there is additional force applied throughout the impact, that will simply add to the amount of total force transferred to the target. If you have ever seen anyone receive an open-handed slap with a follow-through behind it (I am sure you will be able to find many such videos on youtube), then you can appreciate from the sound that it is still a slap, yet clearly transfers more force than a limp-wristed "soap opera slap".
  17. Jul 19, 2011 #16


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    Slap someone with a ruler?

    That's kind of pushing it IMO. I'm not sure I would call a hit with a ruler a 'slap'.
  18. Jul 20, 2011 #17
    A ruler is a great slapping device. IMO
  19. Jul 20, 2011 #18
    I'm certain there's a good comparison to water, but I'm not sure what. If you take a flat object and throw it onto the surface of a body of water just right, it will produce a similar effect where most of the energy is transferred to the surface of the water rather than penetrating the water.

    So I think that the physics of fluid dynamics will be involved in the answer I'm looking for.
  20. Jul 20, 2011 #19


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    No, I think that demonstrates that, if you set up the circumstances just right, sometimes water does not behave traditionally like a fluid - no fluid dynamics needed.
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