he usual answer to this question is that the averagespeed
is 35kmjhr. But this is not true. It would be true if the car
travelled at the speeds given for equal periods oftime, But it is
clear.from the conditions ofthe question that the car takes two
of
differentperiods time (t 1 and t 2) to travel thesamedistancesin
the two directions, since:
S = viti = v 2 t 2
Therefores = 40t
1
= 30t 2)hence
= !
11 t2..
KINEMATICS 87
The average speed
2s 60ta
=
v = -+- = ! + 34·3 km/hr,
av t 1 t 2 t 2 t 2