Question

A train slows down from 80 km/h uniform retardation of 2 m/s². How long will take
attain a speed of 20km/h ?​

Answer 1

Given data:

• Initial Velocity = Vi = 80 km/h
• Final Velocity = Vf = 20km/h
• Acceleration = a. = - 2m/s²

To find:

• Time = t = ?

Solution:

$\mathcal{Vi = 80km {h}^{ - 1} } \\ \mathcal{Vi = \frac{80 \times 1000m}{60 \times 60s} } \\ \mathcal{Vi = 22. {2ms}^{ - 1} }$

$\mathcal{Vf = 20km {h}^{ - 1} } \\ \mathcal{Vi = \frac{20 \times 1000m}{60 \times 60s} } \\ \mathcal{Vi ={5.6ms}^{ - 1} }$

Now, By using First Equation of motion.

$\mathcal{Vf = Vi + at} \\ \mathcal{ \: \: \: \: t = \frac{Vf + Vi}{a} }$

Put Values in the the equation.

$\mathcal{t = \large { \frac{5.6 {ms}^{ - 1} - 22.2 {ms}^{ - 1} }{ - 2 {ms}^{ - 2} } }} \\ \mathcal{t = 8.3s}$

Thus the train will take 8.3 s to attain the required speed.

Answer 2

The upper answer is correct you can follow that answer.