NCERT Class 9 Science Solutions: Chapter 11 – Work and Energy Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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

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

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

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

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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