為什麼在加速度的單位裡，時間單位會出現兩次?

Acceleration is defined as the RATE of change of velocity with time.
That is, acceleration = change of velocity/time
Since the unit for velocity is m/s, the unit for acceleration is thus (m/s) divided by s, this gives (m/s)/s or m/s2
If the acceleration of an object is, say 2 m/s2, the physical meaning of this quantity is that object velocity will increase by 2 m/s for each and every 1 second interval of time.
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如果1架車 10s由吾郁去到180km/h-1,10s內平均速道點計?
Average velocity = (final velocity + initial velocity)/time taken
since 180 km/h = 50 m/s
average velocity = (50+ 0)/10 m/s = 5 m/s
總埋10s內總位移又點計?
Displacement = average velocity x time taken
i.e. displacement = 5 x 10 m = 50 m
You can see that in this example, the acceleration of the car is 5 m/s2. That is to say, the car picks up velocity of 5 m/s for every second lapsed in time. Hence, after 10 seconds, its velocity will be 50 m/s.

速度就是距離隨時間的變化，
所以單位是距離除時間 ：[m/s]
　
而加速度就是速度隨時間的變化，
所以單位是速度除時間=(距離除時間)再除時間：[(m/s)/s=m/s^2]