Answer the following question given this paragraph: 

The force of friction between these two blocks is related to two factors. The first factor is the roughness of the surfaces that are interacting, which is called the coefficient of friction , (Greek letter mu). The second factor is the magnitude of the force pushing the top block down onto the lower block. It is reasonable that the more forcefully the blocks are pushed together, the more difficult it will be for one to slide over the other. The force pushing these blocks together is the result of gravity acting on the top block and pressing it against the bottom block, which resists the weight with an equal and opposite force called the normal force . The force of friction can be calculated by.


Q: The roughness of interacting surfaces and the magnitude of force pushing one object onto another are two factors related to what force?


A:
friction