Question

A bus decreases its speed from 80 km h −1 to 60 km h −1 in 5 s. Find the acceleration of the bus.

The answer follows-


Initial speed ofthe bus,u= 80 km/h

Final speed ofthe bus,v= 60 km/h

Time take to decrease the speed,t= 5 s



Here, the negative sign of acceleration indicates that the velocity of the car is decreasing.

Then why is initial or final velocity getting multiplied by 5/18?? What is the point?

Answer 1

It is multiplied by 5/18 to convert km/h into m/s

Answer 2

In this problem, you will have to change the speeds of bus from km h^-1 to m s^-1 because the time is given in seconds.

Now ,

Initial Speed of Bus , u = 80km h^-1

$= > \frac{80 \times 1000 \: m}{60 \times 60 \: s}$

=> 22.22 m s^-1

Final Speed of Bus v = 60 km h^-1

$= > \frac{60 \times 1000 \: m}{60 \times 60 \: s}$

=> 16.66 m s^-1

And , Time taken , t = 5 s

Now , Acceleration :-

$= > a = \frac{v - u}{t}$

$= > a = \frac{16.66 - 22.22}{5}$

$= > a = \frac{ - 5.56}{5}$

=> a = -1.11 m s^-2

Thus , the acceleration of the bus is , -1.11 m s^-2. The minus sign for acceleration shows that it is actually negative acceleration or retardation.