don't forget the conditions under which you derived it,

不要忘记你是在什么条件下导出的
and don't apply the result to a case

这种情况下不要直接把公式拿来用
where the answer does not belong to the domain

即你所得到的结果
that you assumed you're applying it to.

不在假设的阈值范围之内
So, the trouble with friction is the following.

就会得到如下关于摩擦力的问题
Friction is not a definite force with a definite magnitude or direction.

摩擦力不是一个大小和方向都确定的力
The magnitude is definite,

大小是确定的
but direction is not definite.

但方向是不确定的
It is uphill if you're going downhill;

若沿斜面向上运动力的方向反之向下
it's downhill if you're going uphill.

若沿斜面向下运动力的方向反之向上
It's like the constant opposition to motion.

就好像总是在和运动对着干
There is force of gravity,the normal force,

有些力如重力法向力