The average acceleration a of an object that changes its speed from v_0 to v_f during a time interval t is:
[tex]a=\frac{v_f-v_0}{t}[/tex]Since the car is initially moving at 25.0m/s and it stops in 3.00 seconds, then its final speed is 0m/s.
Replace v_f=0, v_0=25.0m/s and t=3.00s to find the average acceleration of the car:
[tex]a=\frac{0\frac{m}{s}-25.0\frac{m}{s}}{3.00s}=-8.333\ldots\frac{m}{s^2}[/tex]Therefore, the average acceleration of the car when it stops from 25.0m/s in 3.00 seconds, is -8.33m/s^2.
