Select two objects with flat surfaces such as books with different covers, for instance hard back and paperback. You will also need an inclined surface which can be tilted, such as a larger book or a small table.
Form a hypothesis concerning the coefficient of static friction. Does the coefficient of static friction depend on the nature of the surface, for instance the surface of the table top, the surface of the sliding objects, or on both surfaces? Which case corresponds to both objects beginning to slide at the same angle, and which corresponds to the objects beginning to slide at different angles?
Use the angle of repose (θr) and the tangent function, µ =tan(θr), to find the coefficient of static friction in both cases. Was your hypothesis suppored or contradicted by the experiment?