Question

A car starts from rest and accelerates (at a constant rate) to a speed of 76.8 km/h in 8.63 s. The car continues travelling at 76.8 km/h. What is the average speed of the car at 19.22 s, ie. over all the time it has been moving? Give your answer in km/h.

Answer 1

Let us discuss the accelerated motion:
vi = 0 m/s
vf= 76.8 km/hr
t = 8.63 s = 8.63/3600 hr

use:
vf=vi+a*t
76.8 = 0 +a*8.63/3600
a = 32037.1 m/s^2

distance moved during this time is d1.
vf^2 = vi^2 + 2*a*d1
76.8^2 = 0 + 2* 32037.1 *d1
d1= 0.092 km

Let us discuss the constant speed motion.:
time = (19.22-8.63) s = 10.59 s = 10.59/3600 hr

distance during this time , d2 = vf*time
= 76.8*10.59/3600
=0.226 km

Finally:
Total distance = 0.092 + 0.226 = 0.318 km
Total time = (8.63+10.59)/3600 hr
Average speed = total disatnce/ total time
= 0.318*3600 / (8.63+10.59)
= 59.6 km/hr

Answer:  59.6 km/hr