NCERT Solutions for Class 9 Science Chapter 11

Important NCERT Questions

Work and Energy

NCERT Books for Session 2022-2023

CBSE Board and UP Board

Exercises Questions

Page No-158

Questions No-5

# A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.

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1. Horizontal Movement:

– The object is moved horizontally on the table from point A to point B.

2. Force of Gravity:

– The force of gravity acts vertically downward towards the center of the Earth.

3. Direction of Displacement:

– The displacement of the object is horizontal, parallel to the table’s surface, from point A to point B.

4. Perpendicular Relationship:

– The force of gravity and the displacement of the object are perpendicular to each other.

– The angle between the force of gravity and the displacement is (90°).

5. Effect on Work Done:

– According to the work done formula (W = Force x Displacement x cos(θ), where (θ) is the angle between force and displacement:

– When the force and displacement vectors are perpendicular (cos(90°) = 0), the cosine of (90°) is zero.

– Zero cosine means that the work done by the force of gravity is zero.

6. Conclusion:

– When an object is moved horizontally against the force of gravity, such as on a table from point A to point B, the work done by the gravitational force is zero.

Understanding this concept illustrates that when an object moves horizontally (perpendicular to the gravitational force) on a surface like a table, the force of gravity does not contribute to the work done on the object because the displacement is perpendicular to the direction of gravity. Therefore, no work is done by gravity in this scenario.