Question

Q.3 A man walks on a straight road from his home to a market 2.5 km away with a speed of 5 km/h. Finding the market closed, he instantly turns and walk back home with a speed of 7.5 km/h. The average speed of the man over the interval of time 0 to 40 minutes is equal to

Answer 1

From home to market

distance = 2.5 km

speed = 5 km / hr

time

= distance / speed

= 2.5 /5

= 0.5 hr

= 0.5 × 60

= 30 min

time taken by the man to travel from home to market = 30 min

From market to home

distance = 2.5 km

speed = 7.5 m/ s

time taken

= 2.5 / 7.5

= 0.3 hr

= 0.3 ×60 min

= 18 min

total time = 30 + 18 = 48 min

but we need to find the avg speed of the body in the time interval of 0 to 40 min

distance covered by the body in 10 min while going from market to home

= 10/60× 7.5

= 1.25 km

total distance covered in 40 min

= dist covered in 30 min + dist covered in 10 min

= 2.5 +1.25

= 3.75 km

total time =40 min = 40 / 60 = 0.66 hr

average speed

= total distance / total time

=3.75 / 0.66 hr

= 5.68 km / hr

average speed = 5.68 km / hr