Class 9 Science Exploration Solutions English Medium

NCERT Solutions Class 9 Science Exploration English Medium

Work, Energy, and Simple Machines

Topic: Work in Science

Chapter 7. Work, Energy, and Simple Machines

This section introduces the scientific concept of work. In daily life, many activities are called work, but in science, work is done only when a force causes an object to move in the direction of the applied force. This page explains the meaning of work, the conditions required for work to be done, the formula of work, its SI unit, different types of work and their applications in everyday life. The concept forms the foundation for understanding energy and power. 

Work

In science, work is done only when a force acting on an object produces displacement in the direction of the applied force. Simply applying a force without causing displacement is not considered work.

Definition

Work is said to be done when a force applied on an object produces displacement in the direction of the force. 

Conditions for Work to be Done

• Application of Force – A force must act on the object.
• Displacement – The object must move from its original position.
• Direction of Force – The displacement should have a component in the direction of the applied force.

Scientific Formula

Work (W) = Force (F) × Displacement (s)

This formula is applicable when a constant force acts in the direction of displacement. 

SI Unit of Work

The SI unit of work is Joule (J).

One Joule

One joule of work is done when a force of 1 newton moves an object through a displacement of 1 metre in the direction of the applied force.

1 J = 1 N × 1 m 

Factors Affecting Work

• Magnitude of Force – Greater the applied force, greater the work done for the same displacement.
• Displacement – Greater the displacement in the direction of force, greater the work done.

Types of Work

• Positive Work – Work is positive when force and displacement are in the same direction.
• Negative Work – Work is negative when force acts opposite to the direction of displacement.
• Zero Work – Work becomes zero when there is no force, no displacement, or when the force acts perpendicular to the displacement. :contentReference[oaicite:4]{index=4}

Examples of Positive Work

• Pushing a Wheelchair – The applied force and displacement are in the same direction.
• Lifting a School Bag – The upward force moves the bag upward.

Examples of Negative Work

• Goalkeeper Stopping a Football – The applied force is opposite to the direction of motion of the ball.
• Applying Brakes on a Bicycle – The braking force acts opposite to the motion of the bicycle. 

Examples of Zero Work

• Pushing a Wall – Force is applied but the wall does not move.
• Carrying a Box Horizontally – The supporting force is vertical while the displacement is horizontal.

Force–Displacement Graph

For a constant force, the work done is equal to the area under the force–displacement graph. Even when the force changes, the area under the graph represents the total work done. 

Real-Life Applications

• Construction Work – Lifting bricks or cement bags involves mechanical work.
• Transportation – Engines perform work to move vehicles.
• Sports – Players perform work while kicking, throwing or lifting sports equipment.
• Machines – Cranes and elevators perform work by lifting heavy loads.

ATP Education Concept Builder

Many everyday activities are called work, but scientifically they may not involve work. For example, pushing a wall for several minutes makes you tired, yet no scientific work is done because the wall does not move. Always check whether force and displacement occur together before concluding that work has been done.

ATP Education Exam Booster

• Remember the three conditions required for work to be done.
• Learn the formula W = F × s and its SI unit.
• Differentiate clearly between positive, negative and zero work.
• Practise identifying work done in real-life situations.
• Remember that the area under a force–displacement graph represents the work done.
• CBSE competency-based questions often ask whether scientific work is done in everyday activities such as pushing a wall, carrying a bag or applying brakes.

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