Chapter 10

NCERT
Class 9
Science
Solutions
10. An object of 40 kg in raised to a height of 5m 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. (g = 10 ms–2)

Question:

An object of 40 kg in raised to a height of 5m 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. (g = 10 ms–2)

Answer:

Mass of the object m = 40 kg

Height h = 5m

Acceleration due to gravity,

g = 10 ms–2

∴ Potential energy = mgh

                               = 40 kg × 5m × 10 ms–2

                               = 2000J

When the object is half-way down, then the mass of the object = 40 kg

height h = 2.5m

Acceleration due to gravity,

g = 10 ms–2

From the third equation of motion

v2 - u2 = 2as

v2 = u2 + 2as

v2 = 0 + 2 x 10 x 2.5

v2 = 50 ms–1

Kinetic energy K.E. = 1/2 mv2

K.E. = 1/2 x 40 x 50

K.E. = 1000 J

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