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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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