physics---velocity

It is possible.
The sign fixed to the acceleration indicates its direction. Speed is a scalar quantity that has no direction.

In that sense, an object moving in the -x direction under an acceleration would have a -ve acceleration whereas its speed is increasing.

A more conrete example is a pendulum bob (assume its rest position is at the origin) swinging about the origin. The motion of the bob from the extreme right back to the origin would have a -ve acceleration but with speed increasing.

acceleration負數，除了代表減速外，
還代表與原本運動方向相反的加速運動。

acceleration正數，與原來方向一致，並進行加速。
acceleration負數，與原來方向一致，進行減速。
acceleration負數，與原來方向相反，進行加速。

其實，acceleration代表加／減速外，還有方向。
所以acceleration是vector。