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Why is an applied dead load more likely to overcome the normal force of friction on an inclined plane than a live load of equal mass under identical test conditions?

I have tested a wide variety of roofing materials for slip resistance in a range of roof slopes using various boot outsole materials. With all test protocols, equipment, and conditions being equal, why are tests utilizing a 210 lb live load (roofer wearing boots) more than 85% more successful (no motion between contact surfaces) than those utilizing a 210 lb dead load (weight sled on boot outsoles) of equal mass on the same sample at the same pitch? What Newtonian principles are at work here? Thank you.

If you've ever tried to maximize the angle of an inclined plane before a mass on it slips in your physics class, the answer should probably occur to you.  A "live load" has some inherent motion and jiggle to make its contact surfaces slip.  Kinetic friction is, for obvious physics reasons, lower than static friction.  Therefore you're dealing more with a situation where something can shake loose easier and you get to deal with kinetic friction instead of static.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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