Question

A car moves through 20km at a speed of 40km/h, and the next 20km at a speed if 60km/h. calculate its average speed.

Answer 1

fr 1st 20 km ,time= 20/40=1/2hr
fr 2nd 20 km , time= 20/60= 1/3hr
total time taken = 1/2+1/3=5/6hr
so average speed= 40÷5/6=48 km/hr

Answer 2

Let the initial and final velocities of the car are denoted as $v_{1}\ and\ v_{2}\ respectively$

We have been given  $v_{1} =\ 40\ km/hr$

                                    $v_{2} =60\ km/hr$

Let the distance travelled by the car in two different instants are denoted as $s_{1}\ and\ s_{2}\ respectively$

Here $s_{1} =\ 20\ km$

         $s_{2} =\ 20\ km$

The time taken to travel first distance is $t_{1} =\frac{s_{1}} {v_{1}}$

                                                                     $=\frac{20}{40}\ hour$

                                                                     $=0.5\ hour$

The time taken to travel second distance is $t_{2} =\frac{s_{2}} {v_{2}}$

                                                                              $=\frac{20}{60}\ hour$

                                                                               $=0.33\ hour$

We are asked to calculate the average speed of the car.

The average speed of the car is calculated as -

                                     $v_{avg} =\frac{tota\ distance}{total\ time}$

                                            $=\frac{s_{1}+ s_{2}} {t_{1} + t_{2}}$

                                            $=\frac{20+20}{0.5+0.33}\ km/hr$

                                             $=48.19\ km/hr$       [ans]