MadSci Network: Physics

Re: can a dog's claw scratch glass?

Date: Mon Jun 7 23:32:49 2004
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 1084589029.Ph


I presume that your dog is scratching the door glass trying to get in when 
the hardness scale says it can't happen.

Moh's hardness scale rates materials on their ability to scratch each 
other when moved in contact with each other with low impact and moderate 
force.  If a material scratches one material and doesn't scratch another 
it lies between them on the scale.  The values can be refined a bit by 
using intermediate hardness materials but is not an exact method of 
defining hardness.  The scale and some common examples are given in the 
following sites [and many others that show up in a search].

Fingernails and dogs' toenails are about 2.5  on the Moh scale;  glass is 
between 5 and 6 depending on the type.   So a dog's toenail  or your 
fingernail  will not scratch glass.   However, if you scratch a new copper 
penny, hardness 3, with your fingernail and press really hard you will 
find that you don't sctatch it but you do mar the surface and destroy its 
uncirculated condition.  Similarly one penny will scratch another, so 
there is a little ambiguity.  There is a dependence on speed and impact 
force.  More refined hardness measurements such as Vickers, Knoop and  
Rockwell scales for metals, and Shore and IRHD scales for rubber and 
plastics control the force, speed, and time of application.

Of course one says "My dog does scratch the door glass when he tries to 
get in".  He does so probably because of fine sand particles imbedded in 
the toenails,  dirt on the glass, and possibly even because of the high 
impact and the high speed of his claw on the glass.  The dirt is most 
likely the culprit but the force is probably contributing. 

This is one of the gray areas in material science, the prediction and 
prevention of wear when materials encounter each other.  In many cases 
there are real surprises when reasonably good material choices are put 
into practice and fail from friction wear or corrosion. Material choice is 

The answer might be quartz or corundum windows; check their hardness on 
the Moh scale.



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