1.

the speed of bicycle decreased from 18 km/h to 9km/h . if total mass of th cyclist with bicycle is 70 kg . calculate the work done is overcoming the friction offered by the brakes and the road . does the K.E.of the system also reduce to half its initial value ?

Answer»

M = 70 kg
v = 9 kmph = 9 * 5/18 = 2.5 m/s
u = 18 kmph = 18 * 5/18 = 5 m/s

Work done  = CHANGE in kinetic energy
              = initial KE - final KE
              = 1/2 m u² - 1/2 m v²
              = 1/2 * 70 * 5² - 1/2 * 70 * 2.5²
            = 875 J - 218.75 J  = 656.25  J

 The speed has come down to half its VALUE.  But the Kinetic energy becomes 218.75/875 = 1/4 TH of inital KE.



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