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Physics/How do Spiked shoes affect friction?

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Question
My teacher said because the spikes are sharp,thus decreasing the contact area and increase the friction.But my dad said that the area have no effect on the friction force as there is no area in the friction force formulaFr = μN.


Also which one is the correct formula of friction?Fr = μN or Ff= μ*m*g?

Answer
Spikes are sharp so that they can part the wood fibers easily.  
Never use * to indicate multiplication.  Ff = (mu)mg is only correct on a flat surface.  
Ff = (mu)N where N is the normal force (perpendicular to the surface) is more general.  Your dad is correct, but generally, the bigger the mass, the bigger the surface area of contact.  

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Richard J. Raridon

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I can answer most questions in undergraduate physics courses, including electricity and magnetism, atomic and nuclear, mechanics and optics.

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