|MadSci Network: Physics|
Glenn, 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]. http://www.24carat.co.uk/hardnessmohsscale.html http://www.amfed.org/t_mohs.htm 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 important. The answer might be quartz or corundum windows; check their hardness on the Moh scale. Regards, JimG
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