NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 8

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Q 17 Image Showing Kinetic Energy Of Car.

Q 17 Image Showing Kinetic Energy of Car

Question 17:

Q. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

Answer:

Kinetic energy,

Where,

Mass of car,

Velocity of car,

Hence, of work is required to stop the car.

Question 18:

In each of the following a force, is acting on an object of mass, The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.

Q 18 Image Showing Force In Three Cases.

Q 18 Image Showing Force in Three Cases

Answer:

Work is done whenever the given two conditions are satisfied:

  • A force acts on the body.

  • There is a displacement of the body by the application of force in or opposite to the direction of force.

Case I

Q 18 (I) Image Showing Force In Case I.

Q 18 (I) Image Showing Force in Case I

In this case, the direction of force acting on the block is perpendicular to the displacement. Therefore, work done by force on the block will be zero.

Case II

Q 18 (II) Image Showing Force In Case II.

Q 18 (II) Image Showing Force in Case II

In this case, the direction of force acting on the block is in the direction of displacement. Therefore, work done by force on the block will be positive.

Case III

Q 18 (III) Image Showing Force In Case III.

Q 18 (III) Image Showing Force in Case III

In this case, the direction of force acting on the block is opposite to the direction of displacement. therefore, work done by force on the block will be negative.

Question 19:

Q. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Answer:

Acceleration in an object could be zero even when several forces are acting on it. This happens when all the forces cancel out each other i.e., the net force acting on the object is zero. For a uniformly moving object, the net force acting on the object is zero. Hence, the acceleration of the object is zero. Hence, Soni is right.

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