Class 9 Science Notes on Chapter 7 Work, Energy, and Simple Machines

Work, energy, and simple machines are important concepts that help explain how simple tasks are performed in daily life. Whether lifting a load, riding a bicycle, or using tools such as levers and pulleys, these concepts help us understand how force and energy make work easier.

Class 9 Science Notes on Chapter 7 Work, Energy, and Simple Machines cover the meaning of work, different forms of energy, the law of conservation of energy, power, and various simple machines. Students will also learn about mechanical advantage, efficiency, and the practical applications of simple machines in everyday life.

Important Topics Covered in Class 9 Science Notes on Chapter 7 Work, Energy, and Simple Machines

Work Done by a Constant Force	Positive and Negative Work Done
Forms of Energy	Work-Energy Theorem
Kinetic Energy	Mechanical Energy
Gravitational Potential Energy	Potential Energy
Power	Conservation of Mechanical Energy
Mechanical Advantage	Simple Machines
Inclined Plane	Pulley
Law of Conservation of Mechanical Energy	Lever

Complete Class 9 Science Notes on Chapter 7 Work, Energy, and Simple Machines

Work Done by a Constant Force

In everyday life, we often use the word work for any task that requires effort. In science, work has a specific meaning. Work is said to be done when a force acting on an object causes displacement.

For example, when a bag is lifted from the ground, an upward force is applied, and the bag moves in the direction of the force. In this case, work is done on the bag.

The amount of work done depends on:

• The magnitude of the force applied
• The displacement produced in the direction of the force

A larger force acting through the same distance results in more work. Similarly, the same force acting through a larger distance also results in more work.

Work

The work done by a constant force on an object is equal to the product of the force applied and the displacement produced in the direction of the force.

$Work Done = \text{Force} \times \text{Displacement}$

 W=F×s

Where W is Work done, F is Force applied, and s is displacement in the direction of the force.

The SI unit of work is the joule (J).

One joule of work is done when a force of 1 newton causes a displacement of 1 metre in its direction.

1 J=1 N×1 m 

Work Done from a Force-Displacement Graph

The work done by a force can be determined using a force-displacement graph.

For a constant force, the work done is equal to the area under the graph between the initial and final positions.

$\text{Work Done} = \text{Area under the Force-Displacement Graph}$

When is Work Done equal to zero

Work done depends on both force and displacement.

Work done is zero in the following situations:

Condition	Reason
Force is zero (F = 0)	No force acts on the object.
Displacement is zero (s = 0)	The object does not move even though a force is applied.

For example, when you push a rigid wall, the wall does not move. Since there is no displacement, no work is done on the wall, even though you may feel tired.

The feeling of tiredness occurs because the muscles in the body use energy while applying the force.

Positive and Negative Work Done

The sign of work done depends on the directions of the force and displacement.

Positive Work Done

When the force and displacement act in the same direction, the work done is positive.

Negative Work Done

When the force acts in the opposite direction to the displacement, the work done is negative.

The Work-Energy Theorem

An object that has the capacity to do work is said to possess energy. For example, a moving cricket ball can knock down wickets, and a flowerpot raised to a height can do work if it falls. In both cases, the objects possess energy.

An object gains energy when work is done on it. A ball gains energy when it is thrown, and a flowerpot gains energy when it is lifted. This shows that work and energy are closely related.

The Work-Energy Theorem states that:

$\text{Work Done on an Object} = \text{Change in its Energy}$

W=ΔE

Forms of Energy

Energy is the capacity to do work. It exists in different forms, such as mechanical energy, electrical energy, thermal energy, light energy, sound energy, and chemical energy.

Energy can be converted from one form to another. For example, a bulb converts electrical energy into light energy, a water heater converts electrical energy into thermal energy, and a ringing bell converts mechanical energy into sound energy.

Mechanical Energy

Mechanical energy is the energy possessed by an object due to its motion or position. It is mainly of two types:

• Kinetic Energy
• Potential Energy

The energy possessed by an object due to its motion is called kinetic energy. All moving objects, such as a moving bicycle, rolling ball, or running athlete, possess kinetic energy.

An object at rest has zero kinetic energy. When a force acts on an object and sets it in motion, work is done on it. According to the Work-Energy Theorem, this work appears as the kinetic energy gained by the object.

Formula for Kinetic Energy

The kinetic energy of an object is given by:

$K = \frac{1}{2}mv^2$

Where K is kinetic energy (J), m is mass of the object (kg), and v is velocity of the object (m/s).The SI unit of kinetic energy is joule (J).

Potential Energy

The energy possessed by an object due to its position, shape, or configuration is called potential energy.

Potential energy can also be stored due to the relative positions of objects. For example, when a ball is lifted above the ground, work is done against gravity. The ball-Earth system stores energy because of their separation. When the ball is released, this stored energy changes into kinetic energy as the ball falls. 

Gravitational Potential Energy

Gravitational potential energy is the energy possessed by an object due to its position above the Earth's surface. The higher an object is raised, the greater its gravitational potential energy.

U=mgh

Where U is Gravitational potential energy (J), m is the mass of the object (kg), g is Acceleration due to gravity (9.8 m/s² or approximately 10 m/s²) and h is height above the ground (m).

The SI unit of gravitational potential energy is joule (J).

Conservation of Mechanical Energy

The sum of an object's kinetic energy and potential energy is called its mechanical energy.

Mechanical Energy=Kinetic Energy+Potential Energy

When an object falls freely under gravity, its potential energy decreases while its kinetic energy increases. The loss in potential energy is equal to the gain in kinetic energy.

As a result, the total mechanical energy of the object remains constant throughout its motion.

Law of Conservation of Mechanical Energy

When only gravitational force acts on an object and no external forces such as friction or air resistance are present, the total mechanical energy remains constant.

Mechanical Energy=Constant

Power

Power is the rate at which work is done.

$P = \frac{W}{t}$

Where P is power, W is work done, and t is time taken.

SI Unit: Watt (W)

More work done in less time means greater power.

1 watt = 1 joule of work done per second (1 W = 1 J s⁻¹).

Simple Machines

Simple machines make work easier by changing the magnitude or direction of force.

Mechanical Advantage (MA),

$MA = \frac{\text{Load}}{\text{Effort}}$

Pulley

A pulley is a wheel with a groove through which a rope passes.

Important Key Points,

• A fixed pulley changes the direction of force.
• It does not reduce the force required.
• Pulling downward is easier than lifting upward directly.
• Mechanical Advantage of a fixed pulley = 1.

Inclined Plane

An inclined plane is a sloping surface used to move heavy objects to a height with less effort.

Requires less force than lifting an object vertically. The force decreases as the slope becomes less steep. The effort is applied over a longer distance.

Mechanical Advantage of Inclined Plane:

$MA = \frac{L}{h}$

Where L is the length of the inclined plane, and h is the height raised

Important facts to remember,

• Mechanical advantage is greater than 1.
• A longer inclined plane requires less effort.
• Common examples include ramps and stairways.

Lever 

A lever is a simple machine consisting of a rigid bar that rotates about a fixed point called the fulcrum. It helps lift or move heavy loads by applying a smaller effort over a larger distance.

Main Parts of a Lever

Part	Description
Fulcrum	Fixed point about which the lever rotates
Load	The force or object to be moved
Effort	The force applied to move the load
Load Arm	Distance between the load and the fulcrum
Effort Arm	Distance between the effort and the fulcrum

Working of a Lever

When a small effort is applied at one end of the lever, it moves through a larger distance and produces a larger force on the load. The lever makes lifting heavy objects easier by increasing the applied force.

Principle of Lever

The work done on one side of the lever is equal to the work done on the other side.

$F_1 \times d_1 = F_2 \times d_2$

Where, 

•  $F_1$ = Effort
•  $d_1$= Distance moved by effort
•  $F_2$ = Load
•  $d_2$= Distance moved by load

Law of Lever

A lever balances when, 

$\text{Effort} \times \text{Effort Arm} = \text{Load} \times \text{Load Arm}$

Mechanical Advantage of a Lever

Mechanical Advantage (MA) is the ratio of load to effort.

$\text{MA} = \frac{\text{Load}}{\text{Effort}} = \frac{\text{Effort Arm}}{\text{Load Arm}}$

A longer effort arm reduces the effort required to lift the same load.