In each of the following a force, F is acting on an object of mass, m. 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

NCERT Image Q 18.png

Answer

We know that

Work done = Force × Displacement × cos θ

Where, theta is the angle between the force and the displacement

 

Case1

f1.png

The angle between the force and displacement is 90 degrees.

Work done = Force × Displacement × cos θ

        = Force × Displacement × cos 90

        = Force × Displacement × 0

        = 0

 

Hence, work done is 0 .

 

Case 2

f2.png

 

The angle between the force and displacement is 0 degrees.

    Work done = Force × Displacement × cos θ

= Force × Displacement × cos 0

= Force × Displacement × 1

= Force × Displacement

 

Hence, work done is positive .

 

Case 3

f3.png

 

The angle between the force and displacement is 180 degrees.

    Work done = Force × Displacement × cos θ

= Force × Displacement × cos 18 0

= Force × Displacement × (-1)

= - Force × Displacement

Hence, the work done is negative.

 

  1. Class 9
  2. Chapter 11 Class 9 - Work and Energy

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.