The antonyms of *accelerating* are *slowing down* and *decelerating*. *Slowing down* implies an abrupt reduction in speed, while *decelerating* implies a gradual reduction. These words can be used in various contexts such as driving, physics, and sports to describe the reduction in speed or rate of change.

운전자는 빨간불에 접근하면서 *속도를 줄이고* 있었습니다.

運転手は赤信号に近づくと*減速*していました。

The driver was *slowing down* as he approached the red light.

비행기는 활주로에 착륙할 준비를 하면서 *감속*하고 있었습니다.

飛行機は滑走路に着陸する準備をしているときに*減速*していました。

The airplane was *decelerating* as it prepared to land on the runway.