Question

A car travels $40km$ at an average speed of $80km{h}^{-1}$ and then travels $40km$ at an average speed of $40km{h}^{-1}$ . The average speed of the car for this $80km$ trip is

• A. $40km{h}^{-1}$
• B. $45km{h}^{-1}$
• C. $53km{h}^{-1}$
• D. $56km{h}^{-1}$

Answer 1

The correct option is C

$53km{h}^{-1}$

Average Speed

• The average speed is the total distance traveled by the object in a particular time interval.
• The average speed is a scalar quantity.
• It is represented by the magnitude and does not have direction.

Step 1: Given Data

A car travels $40km$ at an average speed $=80km{h}^{-1}$

it travels another $40km$ at an average speed $=40km{h}^{-1}$

Step 2: The average speed of the car for this entire journey

For first $40km$

The time is taken to cover the distance of $40km$ with an average speed of $80km{h}^{-1}$

$\Rightarrow t_1=\frac{dis\tan cetravelled}{averagespeed}$

On putting the given data in the above equation we get,

$\Rightarrow t_1=\frac{40}{80}$

$\Rightarrow t_1=0.5hr$

For another $40km$

The time is taken to cover the distance of $40km$ with an average speed of $40km{h}^{-1}$

$\Rightarrow t_2=\frac{dis\tan cetravelled}{averagespeed}$

On putting the given data in the above equation we get,

$\Rightarrow t_2=\frac{40}{40}$

$\Rightarrow t_2=1hr$

Now for calculating the average speed of the car for this $80km$ trip $=\frac{totaldis\tan ce}{totaltimetaken}$

On putting the calculated data in the above formula we get,

Average speed $=\frac{80}{1.5}\simeq 53km{h}^{-1}$

Hence, option (C) is correct.