mechanics

The sign in front of the [acceleration] represents its direction.

[-a] indicates that the direction of acceleration is pointing towards the -ve X (or -ve Y) direction.

When you compare the direction of [a] with the direction of velocity (v), you could then determine whether the [a] is, in fact, an accleration (i.e. a and v are of same sign) or a retardation (i.e. a and v are both of opposite signs).

其實個acceleration係負係正係睇你自己
如果你設向右為正,,,咁即係左係負
向上為正,,,咁即係下係負等等

例如一個force F ,令物件向上移,acceleration就會係負,
因為原本物件會向下墮, acceleration = 地心吸力,是向下的,acceleration就是正,但向上移的力 F = ma 那個 a 就是負,因為跟地心吸力的方向相反.