# Selina Solutions Class 9 Physics Solutions for Exercise 2A in Chapter 2 - Chapter 2 Motion In One Dimension

Question 44 Exercise 2A

A car is moving with a velocity 20m/s. The brakes are applied to retard it at a rate of 2m/s^2. What will be the velocity after 5s of applying the brakes?

Given: u = 20m/s, retardation= 2m/s^2 , t=5s

If ‘v’ is the final velocity, then as we know,

Acceleration = v-u/t

\begin{aligned} &-2=\frac{v-20}{5} \text { (retardation is negative acceleration) }\\ &\begin{aligned} \Rightarrow & \mathrm{v}-20=-(2 \mathrm{x} 5) \\ \mathrm{v} &=10-20 \\ &=-10 \mathrm{m} / \mathrm{s} \end{aligned} \end{aligned}

The negative sign is an indication that the velocity is decreasing.

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