Question

a car is moving at a speed of 40 km/h. it hits the brakes and slows down at a rate of (-10) km/h for 3 seconds. what is its final velocity?

Explanation:

Step1: Identify the initial - velocity, acceleration, and time

The initial velocity $u = 40$ km/h. First, convert it to m/s. $40\times\frac{1000}{3600}=\frac{40000}{3600}=\frac{100}{9}$ m/s. The acceleration $a=- 10$ km/h/s. Convert it to m/s²: $-10\times\frac{1000}{3600}=-\frac{10000}{3600}=-\frac{25}{9}$ m/s². The time $t = 3$ s.

Step2: Use the kinematic - equation $v=u + at$

Substitute the values into the equation: $v=\frac{100}{9}+(-\frac{25}{9})\times3$.
First, calculate $(-\frac{25}{9})\times3=-\frac{75}{9}$.
Then, $v=\frac{100}{9}-\frac{75}{9}=\frac{100 - 75}{9}=\frac{25}{9}$ m/s.
Convert back to km/h: $\frac{25}{9}\times\frac{3600}{1000}=10$ km/h.

Answer:

10 km/h