Question

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
NCERT Class IX
Sciences - Main Course Book
Chapter 11. Work and Energy

Answer 1

potential energy =mgh
                          40*10*5
                              =2000joules
or '
potential energy =mgh
                               40 *9.8*5
                              =1690joules

Answer 2

1. Potential energy = mgh

PE= 40*10*5

PE= 2000J

2. When the object is halfway down ( s=2.5m), let the velocity be v

Then,

v2-u2=2gs ( v square minus u square)

v2-0= 2*10*2.5

v2= 50

Therefore, kinetic energy of the object

KE= 1/2 mv2= 1/2*40*50= 1000J