Question

A car decreases its speed from $80kmh{r}^{-1}$ to $60kmh{r}^{-1}$ in $5$ seconds. The acceleration of the car is:

• A. $-1.62m{s}^{-2}$
• B. $1.62m{s}^{-2}$
• C. $-1.11m{s}^{-2}$
• D. $1.11m{s}^{-2}$

Answer 1

The correct option is C $-1.11m{s}^{-2}$

Given:
Initial velocity:
$u=80kmh{r}^{-1}$
$u=80\times \frac{5}{18}=22.22m{s}^{-1}$

Final velocity:
$v=60kmh{r}^{-1}$
$v=60\times \frac{5}{18}=16.67m{s}^{-1}$
Time taken, $t=5s$
Let 'a' be the acceleration of the car.
From definiton of acceleration:
$\therefore a=\frac{v-u}{t}$
$\Rightarrow a=\frac{16.67-22.22}{5}$
$\Rightarrow a=-1.11m{s}^{-2}$
$\therefore$ Acceleration of the car is $-1.11m{s}^{-2}$.