Question

A. A car starts from rest and travels for 30 minutes at 50 km/hr. It then stops for 10 minutes before
continuing its journey. It covers the last 25 km in 40 minutes. Draw the distance-time graph for the
journey and answer the following questions :
(1) What is the total distance travelled?
(2) What is the average speed for the whole journey?​

Answer 1

Answer:

1. total dis. travelled= (30÷60)h × 50 + 25=50km

2. av. speed = total dis./total time=

#ans=37.5km/h

Answer 2

Recall:

$\text{Distance} = \text{Speed} \times \text{Time}$

$\text{Speed} = \text{Distance} \div \text{Time}$

$\text{Time} = \text{Distance} \div \text{Speed}$

$\text{Average Speed} = \text{Total Distance} \div \text{Total Time}$

There are 3 parts of the journey:

Part 1 : Travelled for 30 mins at 50 km/h

Part 2: Rested for 10 mins

Part 3: Travelled for 25 km in 40 mins

Find the distance travelled in Part 1:

$\text{Time} = 30 \text{ mins}$

$\text{Time} = \dfrac{1}{2} \text{ hour}$

$\text{Time} = \dfrac{1}{2} \text{ hour}$

$\text{Speed} = 50 \text{km/h}$

$\text{Distance} = \text{Speed} \times \text{Time}$

$\text{Distance} =50 \times \dfrac{1}{2}$

$\text{Distance} =25 \text{ km}$

Find the total distance:

$\text {Distance covered in Part 1} = 25 \text { km}$

$\text {Distance covered in Part 2} = 0 \text { km}$

$\text {Distance covered in Part 3} = 25 \text { km}$

$\text {Total Distance} = 25 + 0 + 25 \text { km}$

$\text {Total Distance} = 50 \text { km}$

Find the total time taken:

$\text {Time taken in part 1} = \dfrac{1}{2} \text { hour}$

$\text {Time taken in part 2} = 10 \text { mins}$

$\text {Time taken in part 2} = \dfrac{1}{6} \text { hour}$

$\text {Time taken in part 3} = 40 \text { mins}$

$\text {Time taken in part 3} = \dfrac{2}{3} \text { hour}$

$\text {Total Time Taken} = \dfrac{1}{2} + \dfrac{1}{6} + \dfrac{2}{3}$

$\text {Total Time Taken} = \dfrac{4}{3} \text{ hours}$

Find the average speed:

$\text{Average Speed} = \text{Total Distance} \div \text{Total Time}$

$\text{Average Speed} = 50 \div \dfrac{4}{3}$

$\text{Average Speed} = 37.5 \text { km/h}$

Answer: Total distance = 50 km, average speed = 37.5 km/h