Answer:
[tex]a=-1.875\ m/s^2[/tex]
Explanation:
Motion With Constant Acceleration
It occurs when the velocity of an object changes uniformly in time.
The equation that rules the change of velocities is:
[tex]v_f=v_o+at[/tex]
Where:
a = acceleration
vo = initial speed
vf = final speed
t = time
Solving for a:
[tex]\displaystyle a=\frac{v_f-v_o}{t}[/tex]
The initial speed is vo=72 Km/h and it's reduced to vf=45 km/h in a time of t=4 s. Both speeds will be converted to m/s:
vo=72*1000/3600 = 20 m/s
vf=45*1000/3600 = 12.5 m/s
Now calculate the acceleration:
[tex]\displaystyle a=\frac{12.5-20}{4}=\frac{-7.5}{4}[/tex]
[tex]\mathbf{a=-1.875\ m/s^2}[/tex]