Question

a car travells at 54 km/h-1 for first 20 seconds, 36 km/h-1 for next 30 seconds and finally 18 km/h-1 for next 10 second find its average speed

Answer 1

d1 = 54*5/18=35/3 t1 = 20
d2=36*5/18=10 t2 = 30
d3=18*5/18=5 t3 = 10
average speed = d1+d2+d3/t1+t2+t3
35/3+10+5/20+30+10
0.2

Answer 2

Hey there!

Formula used:

• Distance = speed * time

• Speed ( in 1 km/h) = 5 /18 (in 1 m/s)

Distance covered when car travels at 54 km/h for first 20 s:

Speed = 54 km / h

           = 54 *( 5/18) m/s

           = 15 m/s

Distance (d₁) = speed * time

                    = 15 * 20 = 300 m

Distance covered when car travels at 36 km/h for  30 s:

S = 36 km/h = 36 * (5/18) m / s  = 10 m/s

d₂ = v * t = 10 * 30 = 300 m

Distance covered when car travels at 18 km/h for  10 s:

S = 18 km/h = 18*(5/18) m/s = 5 m/s

d₃ = v * t = 5 * 10 = 50 m

Average speed = Total distance(d₁ + d₂ + d₃) / Total time

                           = (300 + 300 + 50) m / (20 + 30 + 10) s

                           = (650 m) / 60 s

                           = 10.83 m/s  (←Answer)

Good day!