I have a problem need to be solved here. Someone told me that the friction force must always have the friction coefficient. Is it right or wrong? and we can not calculate the friction force without friction coefficient because this force is only calculated by the coefficient and the Normal force? Is there any way to calculate friction force without coefficient because I think that there are many materials can not identify the specific coefficient? The friction force I think is the force like Newton's Force. So can I calculate the friction force by the second fomula of Newton force: F= m x a????