NCERT Class-9 Science Solutions: Chapter 11-Work and Energy Part 2

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Question 8:

Water stored in a dam possesses

  1. No energy

  2. Electrical energy

  3. Kinetic energy

  4. Potential energy

Answer: D

Question 9:

A body is falling from a height . After it has fallen a height , it will possess

  1. Only potential energy

  2. Only kinetic energy

  3. Half potential and half kinetic energy

  4. More kinetic and less potential energy

Answer: C

Image alf potential and half kinetic energy

Image Alf Potential and Half Kinetic Energy

Image alf potential and half kinetic energy

Short Answer Questions

Question 10:

A rocket is moving up with a velocity . If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?

Answer:

Initial velocity

Initial kinetic energy

Final kinetic energy (K.E.)

Question 11:

Avinash can run with a speed of against the frictional force of , and Kapil can move with a speed of against the frictional force of . Who is more powerful and why?

Answer:

Given, force applied by Avinash = 10 N

Speed of Avinash = 8 ms-1 ‘

Power of Avinash

Now, force applied by Kapil

Speed of Kapil

The power of Kapil

Since, Avinash has more power W than Kapil. Therefore, Avinash is more powerful.

Question 12:

A boy is moving on a straight road against a frictional force of . After travelling a distance of he forgot the correct path at a roundabout (Figure) of radius . However, he moves on the circular path for one and half cycle and then he moves forward up to . calculate the work done by him.

Image the round about

Image the Round About

Image the round about

Answer:

He moves on the circular path for one and half cycle .he forgot the correct path at a round about. He moves on the circular path for one and half cycle so the displacement

So total displacement

We have,

Therefore,