Right.

是的
That's correct.

对
All the forces I drew here,

我这里标注的所有的力
they're not going to change their mind when you change the body's motion.

都不会因为你改变物体运动状态而改变
Gravity is always going to pull down.

重力总是竖直向下
So, all these forces are fixed in magnitude and direction,

所有这些力的大小与方向都是确定的
and therefore once you got a formula for positive a,

因此如果所得方程解为正的 a 的表达式
you can apply it to negative a,

那么这个式子对负的 a 也同样适用
but you cannot do it when you have friction

但当有摩擦力时就不行了
because in friction, you assume the direction of motion.

因为有摩擦时你需要假定运动方向
All right, so now I'm going to leave the plane.

好了现在结束这个斜面的学习
This is it.

就是这样