science
posted by arthur .
a truck is travelling along a straight road at 72 km/hr.The driver saw a woman crossing and immediately steps on the brake.If the truck stops in 7 sec.what is its acceleration

We will assume that the question is asking for the answer in m s^2. I would also draw a picture and define the positive direction as the direction of travel. Note that the symbol for time is s not sec.
First convert the velocity km hr^1 to m s^1
72 km h^1 is
72 x 1000 m km^1 km hr^1 or
72000 m hr^1
3600 s hr^1 so
velocity is
72000 m hr^1/3600 s hr^1 =
20 m s^1
If the truck goes from 20 m s^1
to 0 m s^1 in 7 s then the rate of change of velocity (acceleration) is
give by
(final velocityinitial)/t
=(0 m s^1  20 m s^1)/7 s
= 20 m s^1/7 s