Question

a car travels at 54km/h for first 20s.36km/h for next 30s and finally 18 km per hour for next 10 seconds find its average speed ​

Answer 1

54 km/hr = 54 x 5/18 = 15 m/s

36 km/hr = 36 x 5/18 = 10 m/s

18 km/hr = 18 x 5/18 = 5 m/s

1st Case ---

Speed = 15m/s

Time = 20 s

Distance = Speed x Time = 15 x 20 = 300 m

2nd Case ---

Speed = 10 m/s

Time = 30 sec

Distance = 10 x 30 = 300 m

3rd Case ---

Speed = 5 m/s

Time = 10s

Distance = 5 x 10 = 50 m

Now Total Distance = 300 + 300 + 50 = 650 m

Total time = 20 + 30 + 10 = 60 s

Average Speed = 650/60 = 65/6 m/s

Hope This Helps You!

Answer 2

Answer-

Average speed of the car is 65/6 m/s

$\rule{100}2$

Explanation-

Case 1)

A car travels at 54km/hr for first 20s.

Here, Speed of car is 54 km/hr and in 20 sec i.e. time we have to find it's distance.

Distance = Speed × time

Before substituting the value convert km/hr into m/s. To convert km/hr into m/s multiply with 5/18.

⇒ 54 × 5/18

⇒ 15 m/s.

Now, put the values.

⇒ 15 × 20

⇒ 300 m

Case 2)

Car travels 36 km/hr for 30 sec.

Here, Speed of car is 36 km/hr and in 30 sec i.e. time we have to find it's distance.

Distance = Speed × time

Before substituting the value convert km/hr into m/s. To convert km/hr into m/s multiply with 5/18.

⇒ 36 × 5/18

⇒ 10 m/s

Now, put the values

⇒ 10 × 30

⇒ 300 m

Case 3)

Car travels 18 km/hr for 10 sec.

Here, Speed of car is 18 km/hr and in 10 sec i.e. time we have to find it's distance.

To convert km/hr into m/s multiply with 5/18.

⇒ 18 × 5/18

⇒ 5 m/s

Now,

⇒ 5 × 10

⇒ 50 m

Now, Average speed = Sum of all distances/Total time taken.

In case first, the car travels 300 m and in second case, car travels 300 m and in third case, car travels 50 m.

⇒ 300 + 300 + 50

⇒ 650 m

In case first, car takes 20 sec, in case second, car takes 30 sec and in case third, car takes 10 sec.

⇒ 20 + 30 + 10

⇒ 60 sec

Susbtite the values in above formula

⇒ 650/60

⇒ 65/6 m/s