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    Question

    A constant retarding force of 60 N is applied to a body

    of mass 25 kg moving initially with a speed of 12 m s–1. How long does the body take to stop?
    A 5 sec Correct Answer Incorrect Answer
    B 2 sec Correct Answer Incorrect Answer
    C 3 sec Correct Answer Incorrect Answer
    D 4 sec Correct Answer Incorrect Answer

    Solution

    Given that Retarding force, F = – 60 N Mass of the body, m = 25 kg Initial velocity of the body, u = 10 m/s2 Final velocity of the body, v = 0 Using Newton’s second law of motion, the retardation (a) produced in the body can be calculated as: F = ma – 60 = 25 × a ∴ a = −60/25 a = − 2.4 m/s2 Using the first equation of motion, the time (t) taken by the body to come to rest can be calculated as v = u + a t Therefore, t = -u/a = -12/-2.4 = 5 sec

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