Question

A car travel 30kilometer at a unifrom speed of 40 kilometres and 30km at a uniform speed of 20 km /hr .find its average speed

Answer 1

Given :

▪ A car travels first 30km of total journey at a uniform speed of 40kmph and second 30km at a uniform speed of 20kmph.

To Find :

▪ Average speed of the car.

Solution :

→ Average speed of body is defined as the ratio of total distance travelled to the total time taken.

→ It is a scalar quantity.

→ It has only magnitude.

✒ If body covers first half of total distance at a speed of V1 and second half of total distance at a speed of V2, then average speed of body is given by

☞ Vav = 2V1V2/(V1+V2)

✏ Vav = (2×40×20)/(40+20)

✏ Vav = 1600/60

✏ Vav = 26.67kmph

Formula Derivation :

✒ Let body covers first d distance at a speed of V1 (time t1) and second d distance at a speed of V2 (time t2)

• Total distance = 2d
• Total time = t1 + t2

⏭ Vav = (2d)/(t1 + t2)

⏭ Vav = 2d/d[(1/V1) + (1/V2)]

⏭ Vav = 2/[(1/V1) + (1/V2)]

⏭ Vav = 2V1V2/V1+V2

Answer 2

$\huge\sf{Answer:}$

☆ Given:

⇏ A car travel 30kilometer at a uniform speed of 40 kilometres and 30km at a uniform speed of 20 km/hr.

☆ Find:

⇏ Find it's average speed.

☆ According to the question:

⇏ Distance travelled = 30 km

⇏ Speed = 40 km, 30 km, 20 km.

⇏ Let us assume S1 as the distance of first half and S2 the distance of second half and S12 the average speed.

☆ We know that:

⇏ $\sf S12 = \dfrac{2 \times S1 \times S2}{S1+S2}$

☆ Calculations:

⇏ $\sf S12 = \dfrac{2 \times 40 \times 20}{40+20}$

⇏ $\sf S12 = \dfrac{1600}{60}$

⇏ $\sf S12 = 26.67 \: km/hr$

Therefore, 26.67 is the average speed.