Question

A car travels at 54 km/hr for first 20 secs , 36 km/hr for next 30 secs and finally 18 km/hr for next 10 secs. Find its average speed.

Answer 1

Step 1, Given data

Speed = 54 km/hr = 54 x $\frac{5}{18}$ = 15 m/s, Time = 20 sec

Speed = 36 km/hr = 36 x $\frac{5}{18}$ = 10 m/s, Time = 30 sec
Speed = 18 km/hr = 18 x $\frac{5}{18}$ = 5 m/s, Time = 10 sec

Step 2: Formula used

Average velocity = $\frac{Totaldistance}{Totaltime}$

Step 3: Finding total distance

1st Case ---
Speed = 15 m/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 = 10 s
Distance = 5 x 10 = 50 m

Now Total Distance = 300 + 300 + 50 = 650 m
Total time = 20 + 30 + 10 = 60 s
Step 4: Finding average speed
Average Speed =$\frac{Totaldistance}{Totaltime}=\frac{650}{60}=10.83$ m/s

Hence the average speed is 10.83 m/s.