# science

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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

• science -

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 velocity-initial)/t

=(0 m s^-1 - 20 m s^-1)/7 s

=- 20 m s^-1/7 s

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