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

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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?

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.

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

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

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