I did an experiment the other day in my Grade 11 Physics class. We were measuring the force required to move a few different objects. One object was this block; one side was two pucks, and the other side, was a piece of lumber. We used a dynamometer and we recorded the force just before the object moved. These are the results/dimensions of the objects:

(in this case, the force reading is right before movement, so the force exerted by me = force of friction)

rubber side : area = 88,36cm^2, mass = 890,2g, friction = 7,22N

wood side : area = 91,14cm^2, mass = 890,2g, friction = 3,32N

I am trying to figure out what caused the rubber side's friction to be so much higher. I'm thinking either the rubber is better at gripping to the surface (laminated table) or the rubber side is smaller, so there is more mass per square cm, causing more friction, or both, or neither of those hypotheses.

The problem with that second hypothesis is that we measured the friction for a block of wood, about the same size as the other chunk of lumber, but with a smaller mass. Yet, the lighter block had about the same amount of friction. Here are the results/dimensions for the lighter block:

small wood: area = 85,68cm^2, mass = 197,6g, friction = 3,10N

I can't, for the life of me, figure out what is going on with the experiment. I saw a few suggestions like the wood is more lubricated than rubber, the rubber is more malleable (at a microscopic level, anyway) that it "fits" into the other surface better, etc. I don't know which is right, or if some kind of human error is involved.

P.S. Sorry, not all my units are SI, I figured that having "cleaner" numbers makes it a bit easier to visualize. These are all significant digits.