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      Question

      A constant retarding force of 20 N is applied to a body

      of mass 10 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 7 sec Correct Answer Incorrect Answer
      C 4 sec Correct Answer Incorrect Answer
      D 6 sec Correct Answer Incorrect Answer

      Solution

      Given that Retarding force, F = – 20 N Mass of the body, m = 10 kg Initial velocity of the body, u = 12 m /s 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 – 20 = 10 × a ∴ a = −20/10 = − 2 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 + at ∴ t = -u/a =  −12/−2 = 6 seconds Hence time taken by the body to come to rest is 6 seconds

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