Q36 of 146 Page 145

Calculate the work done by the brakes of a car of mass 1000 kg when its speed is reduced from 20 m /s to 10 m /s?

Mass of the car, m = 1000kg

Initial velocity (v1) = 20m/s


Final velocity (v2) = 10m/s


Initial kinetic energy = 1/2 mv12


= 1/2 x 1000 x (20)2


= 200,000J


Final kinetic energy=1/2 mv22


= 1/2 x 1000 x (10)2


= 50,000 J


Work done by the brakes = change in kinetic energy


= (K.E) 2 - (K.E) 1


= 50,000 - 200,000


= - 150000J


= - 150KJ


(The negative sign shows that the force applied by the breaks is in the opposite direction to the direction of motion of the body)


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