Question

A car travels st 54 km/h for first 20 s, 36 km/h for next 30 s and 18 km/h for next 10 s. Find its average speed.

Answer 1

Hey.

Here is the answer.

distance travelled in first 20s

= 54kmph × 20s

= 54×5/18 m/s ×20s

= 15 m/s × 20s

= 300 m

distance travelled in next 30s

= 36kmph×30s

= 36×5/18 m/s ×30s

= 10 m/s × 30s

= 300 m

distance travelled in next 10s

= 18kmph ×10s

= 18×5/18 m/s × 10s

= 5 m/s × 10s

= 50 m

So, total distance travelled = 300+300+50

= 650 m

total time taken = 20+30+10 s

= 60 s

so, average speed = 650/60 m/s

= 10.83 m/s or 65/6 m/s

= 65/6 × 18/5 kmph

= 39 km/h

Thanks.

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!