my mcat professor said that for a box with mass of X, and want to calculate Fs we can plug in mg for normal force. why does he say that the relationship between mg and mg is that they are the same. thanks can anyone clear this up
Because on any non moving object, there are two forces. Gravity pointing down, and an equal and opposite Normal force pointing up.
F= ma = 0 in this case.
Frictional force is directly proportional to Normal force.
Think of it like this, If you push down on a book and try to move it forward, its harder than if you were to move it without any added normal force. There's added friction that makes it harder to move the book in the first scenario