In this experiment, students used an inclined plane to rank the coefficients of static friction, μS, for various materials. The students used the force of gravity to move a block of wood rather than pulling on the block themselves. A block of wood is placed on a board and the board is slowly raised on the diagonal until a certain angle, θ, is reached. This angle is the point at which the block slides down the board. Students used a smooth board, covered alternately with various substances: bare wood, plastic, aluminum, cotton fabric, glass, and sandpaper for the testing surface. They predicted each surface’s relative coefficient of friction, with 1 being the lowest μS and 6 being the highest. A block of wood was placed on the bare, smooth wood board and one end was raised until the block began to slide. The angle is known as the angle of slip and it was measured using a protractor and recorded in the data table. This procedure was repeated, with each of the other five materials covering the board. Data were recorded in the data table. Students conducted another experiment to determine if the mass of an object affected friction, assuming the types of surfaces remained constant. A spring scale was attached to a wooden block (1 kg). The block was placed on a wooden board and the spring scale was used to pull the block. The magnitude of the force required to start the block moving was recorded in the data table. The experiment was repeated using two, three, and four blocks.
Block (kg) FN (N) Ff (N)
1 9.8 4.95
2 19.6 9.85
3 29.4 14.70
4 39.2 19.70
FN = weight
Ff - friction force
Question:

Consider the two experiments conducted by the students. What were the factors that influence the amount of friction between the block and the surface? According to correct scientific terminology, what general label would you give to these factors?