You can see from this equation that the friction force is larger when Cf is larger. Cf is why you have to push a rubber block harder than a wood block. Rubber sliding on anything has a higher Cf than wood sliding on that same material. Fv, the vertical force, is why it takes more force to slide a lead block than a wood one. The lead is heavier and so Fv is bigger than with a wood block of the same size.
Rubber has a relatively large friction coefficient when tested on most surfaces.
Let's say it's 0.8. If the rubber block weighs 1 pound, then the vertical force is 1 pound, and it's going to take 0.8 pounds of force to push the block at a steady speed. Let's add some vertical force to the block. We could just place a piece of lead on top of the wood block but that wouldn't be very interesting. Instead, let's put an upside-down wing on top of the block, blow some air over the wing, and produce a downforce of 9 pounds.