# NCERT Question 4 - Chapter 11 Class 9 - Work and Energy

Last updated at April 16, 2024 by Teachoo

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Class 9

Chapter 11 Class 9 - Work and Energy

Last updated at April 16, 2024 by Teachoo

NCERT Question 4 Certain force acting on a 20 kg mass changes its velocity from 5 m s–1 to 2 m s–1. Calculate the work done by the force. Mass of the body = 20 Kg Initial velocity = u = 5 m/s final velocity = v = 2 m/s Work done = Change in kinetic energy Initial Kinetic energy = 1/2 mu2 = 1/2 × 20 × 52 = 10 × 25 = 250 J Final Kinetic energy = 1/2 mv2 = 1/2 × 20 × 22 = 10 × 4 = 40 J Now, Work Done = Change in kinetic energy = Final kinetic energy − Initial kinetic energy = 40 − 250 = − 210 J ∴ Work done = − 210 J