An acceleration

The car goes on a straight road at a speed of 72 km/h. At some point, the driver starts to brake and stops the car in 5 seconds. Find: (a) the acceleration during braking and (b) the distance traveled during braking.

Correct answer:

a =  -4 m/s²
s =  50 m

Step-by-step explanation:

$$\begin{gathered} v_1=72\text{km/h}\to \text{m/s}=72:3.6\text{m/s}=20\text{m/s} \\ {v}_2=0\text{m/s} \\ t=5\text{s} \\ a=\frac{v_2-v_1}{t}=\frac{0-20}{5}=-4{\text{m/s}}^2 \end{gathered}$$

$$\begin{gathered} s=v_1\cdot t+\frac{1}{2}\cdot a\cdot t^2=20\cdot 5+\frac{1}{2}\cdot (-4)\cdot 5^2=50=50\text{m} \\ \text{ Verifying Solution: } \\ {s}_0=v_1\cdot t=20\cdot 5=100\text{m} \\ {s}_0>s \end{gathered}$$

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