Question

The brakes of your car can create a deceleration of $5m/s^2$. If you’re moving at 40 m/s on a straight highway and see a​ sign for a speed limit of 20 m/s, in how much time can you reduce the speed to match the limit?

• A. 2 s
• B. 4 s
• C. 6 s
• D. 8 s

Answer 1

The correct option is B 4 s

$a=\frac{v-u}{t}$ ...(i)
Here,
$a=-5m/s^2$ (deceleration)
Initial velocity, u = 40 m/s
Final velocity, v = 20 m/s
By substituting these values in equation (i), we get,
$\Rightarrow -5=\frac{20-40}{t}$
$\Rightarrow t=\frac{20-40}{-5}$
$\Rightarrow t=\frac{-20}{-5}=4s$