Question

A girl goes to school by bus at a speed of 20 km/hr and returns back through the same route at a speed of 60 km/hr. What is her average speed.

Answer 1

Given :

➳ A girl goes to school at a speed of 20kmph and returns back at a speed of 60kmph.

To Find :

➔ Average speed of bus.

Concept :

⇒ Average speed is defined as the ratio if total distance travelled to the total time taken.

⇒ It is a scalar quantitu having only magnitude.

⇒ It can't be negative or zero.

⇒ SI unit : m/s

➢ If a particle moves in two equal intervals of time at different speed v$_1$ and v$_2$ respectively, then average speed is given by

$\bigstar\:\underline{\boxed{\bf{v_{av}=\dfrac{2v_1v_2}{v_1+v_2}}}}$

SoluTion :

$\:\implies\tt\:v_{av}=\dfrac{2v_1v_2}{v_1+v_2}\\ \\ \implies\tt\:v_{av}=\dfrac{2\times 20\times 60}{20+60}\\ \\ \implies\tt\:v_{av}=\dfrac{2400}{80}\\ \\ \implies\underline{\boxed{\bf{v_{av}=30\:kmph}}}\:$

Answer 2

▪ Given -

• A girl goes to school by bus at a speed of 20km/hr and returns back through the same route at a speed of 60km/hr
• 1st average speed = 20km/hr
• 2nd average speed = 60km/hr

▪ To Find -

• Average speed of bus in both two cases

▪ Solution -

• When an object moves in 2 different speed or time, Formula calculated is -

★ $av = \frac{2 \: v1 \times v2}{v1 + v2}$

Placing Values according to given conditions,

$⟹av = \frac{2 \ \times 60 \times 20}{60 + 20}$

$⟹ av = \frac{2400}{80}$

[tex]⟹ av = \frac{\cancel{2400}}{\cancel{80}} [/tex]

[tex]⟹ av = 30km/hr [/tex]

⛬ Average speed of car is 30km/hr