Chapter 9

NCERT
Class 9
Science
Solutions
3. The kinetic energy of an object of mass, m moving with a velocity of 5 ms–1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Question:

The kinetic energy of an object of mass, m moving with a velocity of 5 ms–1 is 25J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Answer:

Mass of the object = m, velocity of the object = 5ms-1, kinetic energy Ek = 25J

and EK = 1/2 mv2

So, 25 = 1/2 (5)2m

m = (25 x 2)/25

m = 2 kg

Case (i) when the velocity is doubled.

Mass of the object = m = 2 kg

And, v = 2 × 5ms-1 = 10ms-1

∴ Kinetic energy (EK) = 1/2 mv2

(EK) = 1/2 x 2 x 102

EK = 100J

Case (ii) when the velocity is increased three times

i.e., v = 3×5

v = 15ms-1

m = 2 kg

Kinetic energy (EK) = 1/2 mv2

(EK) = 1/2 x 2 x 152

(EK) = 225J

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