# NCERT Question 10 - Chapter 11 Class 9 - Work and Energy

Last updated at June 8, 2019 by Teachoo

Last updated at June 8, 2019 by Teachoo

Transcript

NCERT Question 10 (Method 1) An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down. At Height = 5 m Mass of the object = 40 kg Height of the object = 5 m Acceleration due to gravity = g = 10 m/s2 Potential Energy = mgh = 40 × 10 × 5 = 2000 J Object falls half way Object falls half way from height 5 m to 2.5 m We need to find its Kinetic Energy First, lets find velocity When ball drops, Initial velocity = u = 0 m/s Distance Travelled = s = 2.5 m Acceleration = a = Acceleration due to gravity = 10 m/s2 We need to find Final velocity v, We know u, a, and s, finding v using 3rd equation of motion v2 – u2 = 2as v2 – 02 = 2 × 10 × 2.5 v2 = 50 Now, Kinetic Energy at height 2.5 m = 1/2mv2 = 1/2 × 40 × 50 = 1000 J NCERT Question 10 (Method 2) An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down. At Height = 5 m Mass of the object = 40 kg Height of the object = 5 m Acceleration due to gravity = g = 10 m/s2 Potential Energy = mgh = 40 × 10 × 5 = 2000 J Object falls half way Object falls half way from height 5 m to 2.5 m We need to find its Kinetic Energy We will do this by Conservation of Energy Potential Energy at height 5m = 2000 J Kinetic Energy at height 5m = 0 J (Since there is no velocity at the top) At height 2.5 m Potential Energy = m × g × h = 40 × 10 × 2.5 = 40 × 10 × 5/2 = 1000 J By law of conservation of energy, Total energy at height 5 m = Total energy at height 2.5 m Potential energy at 5 m + Kinetic energy at 5 m = Potential energy at 2.5 m + Kinetic energy at 2.5 m 0 + 2000 = 1000 + Kinetic energy at 2.5 m Kinetic energy at 2.5 m = 2000 − 1000 Kinetic energy at 2.5 m = 1000 J Kinetic energy when mass is halfway down is 1000 J Kinetic energy when mass is halfway down is 1000 J

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Class 9

Chapter 11 Class 9 - Work and Energy

About the Author

Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.