It's true the net force is 0, but it doesn't mean you can ignore it.

所受的合力是 0,但是并不意味着你可以忽略它

耶鲁公开课 - 基础物理课程节选

F = ma is universally true, independent of the nature of the force acting on a body.

= ma 是一条普适定律,与作用在物体上的力的属性无关

耶鲁公开课 - 基础物理课程节选