Relationship between friction and weight

So I know that weight correlates with friction, as in more weight of an object means that it will experience more friction. However, what is not clear to me is why this occurs on a molecular level.

Thanks for the help

berkeman
Mentor
So I know that weight correlates with friction, as in more weight of an object means that it will experience more friction. However, what is not clear to me is why this occurs on a molecular level.

Thanks for the help

Welcome to the PF.

Actually, the coefficient of friction does not change much with the normal force (weight of an object). The retarding force due to that friction is proportional to the normal force, F=μN, but the value of μ does not change much for a given material combination.

http://en.wikipedia.org/wiki/Friction

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To answer your question the reason why friction, to an approximation, depends on the normal force is because, for relatively small forces, the surface area in contact increases linearly with the normal force. The root cause of friction is due to asperities. As you can see in the image, as the asperities are compressed the surface area of contact increases.