In a recent physics class I took, my teacher explained how friction is not affected by surface area, but by the "bumpiness" of the two objects and the mass, as well as other things, but not the surface area. But this made me wonder how nails get harder to pull out the deeper they are embedded in a surface. Since the only thing apparently increasing is the contacting surface area of the object, why does it become harder to remove? Also, the definition of inertia is "a tendency for an object to remain in motion while in motion or to stay still while still", would the nail technically have a larger inertia in the previous example? I know that Inertia is solely dependent on mass, but shouldn't this example warrant a better definition? Thanks in advance.