Question

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

Answer 1

average speed=total distance/total time

distance travelled during first 20s is speed ×time

v1=54kmph×5/18=15m/s

distance= 15(20)=300m

distance travelled in next 30s is speed ×time

v2=36kmph×5/18=10m/s

distance = 10(30)=300

distance travelled in the next 10s is speed ×time

v3=18kmph×5/18=5m/s

distance =(5)(10)=50m

average speed =300+300+50/60

=650/60m/s

Answer 2

let displacement be “s”

given that,

Initial speed of the car (u)=54 km/hr=15 m/s

after t=1 sec

Final speed (v)=63 km/hr=17.5 m/s

We know that,

Acceleration= change in velocity per unit time

=(17.5–15)/1

=2.5 m/s^2

By equation of motion,

v^2- u^2= 2*a*s

(17.5)^2- (15)^2=2*2.5*s

306.25–225=5*s

s=16.5