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Help with pressure differences-sponge and wood

  1. Apr 9, 2007 #1
    hi,i have been doing experiments with individual pressure sensors, while attaching the sensor to my thumb, if i press my thumb onto a hard surface the pressure recorded is very high, but when i push my thumb onto a soft surface, like a sponge i have to press a lot harder to get a high pressure reading. newtons third law states that every action force has an equal and opposite reaction force, even though i feel im exerting more pressure when im pushing the sponge, it does not show this on the pressure readings. Do the varying pressure readings have something to do with the elasticity of the sponge, thus increaseing surface area-pressure disribution is higher??
    im a little lost as how to explain this well, sorry.
    Any help would be great. Thanks
  2. jcsd
  3. Apr 9, 2007 #2


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    Are these force or pressure transducers?

    The compliance of the sponge means that your hand has to travel a farther distance until the sponge creates the required reaction force. Think of the sponge as a spring and [tex]F=kx[/tex]. Really you should not have to exert more force to get an equivalent reading. However, if this is indeed a pressure sensor, there may be an issue with the compliance at the measurement location of the transducer. Since the surface is not completely rigid, you may get some deviation.
  4. Apr 9, 2007 #3
    thanks for your message. So if i push down on both the hard surface and soft surface equally, i will get a lower pressure reading with the soft surface as i have to push down further with the soft surface in order to equal the reaction force of the hard surface? is that correct? thanks again
  5. Apr 9, 2007 #4


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    If you push down with an equal amount of force, you should get the same reading from your transducer. The key words are "with the same force". If you are applying the same force that means the object you are pushing against is providing the appropriate reaction force. The sponge will make you move your hand farther than the hard material.

    There may be a bit of a difference if, at the surface of the transducer, there is a bit of compliance that you can not control. Again, that goes back to if you are using a pressure or force transducer. If you are using a force transducer there shouldn't be a difference.
  6. Apr 9, 2007 #5
    hi. It is a pressure transducer. i think i understand what is being said. So what would happen if i didnt push my hand in as far into the sponge? the readings would be less compared to hard surface?
  7. Apr 9, 2007 #6


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    Staff: Mentor

    Is your finger physicallly touching the sponge? That is the only possible way that you could measure less pressure but apply more force.
  8. Apr 9, 2007 #7
    Incase of sponge, the area that experiences the force is more and that is why pressure is less. Press the sponge to such an extent that it will not deform and then your pressure reading should be same with that of hard surface.
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