Calculate the force of friction for a 5kg aluminum block being pulled with constant velocity (uniform motion) across a horizontal steel plank ...?

The purpose of this lab is to determine whether the surface of an area would affect the coefficient of Friction. My classmates and I have learned a lot in this lab and that there could have been some errors in our lab because the strength of how a person pulls it might be a slight different than the normal force. I learned from this lab that the surface area would have no effect on the coefficient of friction.