Work and energy Class 9 Science Chapter 11 NCERT Notes

Class 9 Short Notes on Work and Energy

Introduction

Life activities require energy, which is provided by food. Energy is needed for thinking, playing, singing, reading, writing, jumping, cycling, and running.

Work
An object does work when a force is applied to it, moving it in the force's direction.
Why, despite working hard, does one not always accomplish much?
The product of displacement in the force's direction and force is known as work. In other words, two requirements must be fulfilled in order for work to be done:

  1. It is necessary to exert force.
  2. In the force's direction, there must be displacement.
In some cases, despite your best efforts, you may not accomplish much work as defined by science.

Here are a few instances that demonstrate this:

1. Pushing a Wall: You may become extremely exhausted if you push against a wall with all of your strength. On the other hand, there is no displacement if the wall remains stationary. As a result, no scientifically sound work is done.

2. Holding a hefty Object: You may get fatigue if you stand motionless while holding a hefty baggage. However, according to physics, no work is done because there is no displacement or movement.
The Scientific View of Work

When a force is applied to an item and the object moves in the force's direction, work is done.

  • The force exerted on the item is denoted by Force (F).
  • The distance an object moves in the force's direction is called displacement (s).
Workplace Conditions
For the task at hand:
  1. There must be force.
  2. Displacement in the force's direction is required.
Unit of Work 
  • Unit of work is Newton metre or Joule.
One Joule of work is produced when a body is moved one metre in its own direction by a force of one Newton.

• The following variables affect how much work is completed:
(i) Force magnitude: The more work, the larger the force, and vice versa.
(ii) Displacement: The amount of work increases with displacement and vice versa.

Positive, Negative, and Zero Work

Positive Work :

When an object's displacement and the force applied to it are both in the same direction, positive work is produced. . For instance, you are performing positive work on a box when you push it and it moves in the direction of your push.

Negative Tasks : 
When an object's displacement and the force applied to it are in opposing directions, negative work occurs. The friction force, for example, performs negative work when you attempt to stop a moving car by applying the brakes because it acts against the direction of the car's motion.

Zero Work
Zero work is done when the force applied to an object does not cause any displacement, or when the force and displacement are perpendicular to each other. For example, if you carry a heavy bag while walking on a flat surface, the force you apply (upward) and the displacement (forward) are perpendicular, resulting in zero work.

Energy
  • Energy is the ability to do tasks.
  • The amount of work a body can do is determined by its energy. A body may accomplish more work if it has more energy.
  • Energy is a scalar quantity
  • Unit : The SI unit of energy is Joule (j) and its bigger unit is kilo joule (kj)
Forms of Energy
Energy forms include kinetic energy, potential energy, heat energy, chemical energy, electrical energy, light energy, sound energy, and more. Nuclear power

Kinetics Energy 
The energy that an object possesses as a result of motion is known as kinetic energy. An object's kinetic energy increases with its speed. Kinetic energy can be calculated using the following formula:

Where
  • ( KE ) is the kinetic energy,
  • ( m ) is the mass of the object,
  • ( v ) is the velocity of the object.
Kinetic Energy Examples
Car Moving: Because of its mobility, an automobile on the road has kinetic energy.
Water in Motion: The water in a river has kinetic energy.
Potential Energy
Potential energy is the energy that a body possesses as a result of its position or shape change.
Formula: The gravitational potential energy (P.E.) of an object of mass ( m ) at a height ( h ) from the ground is given by:
  • where ( g ) is the acceleration due to gravity.
Examples :
(i) Water kept in dam : It can rotate turbine to generate electricity due to its position above the ground.

(ii) Wound up spring of a toy car : It possess potential energy which is released during unwinding of spring. Therefore, toy car moves.

Potential energy-influencing factor
  • Height (h): An object's potential energy increases with altitude above the earth.
  • Mass (m): The potential energy of an object increases with its mass.
  • Gravitational Field Strength (g): Potential energy is influenced by the strength of gravity, which on Earth is roughly 9.8 m/s².
POTENTIAL ENERGY OF AN OBJECT AT A HEIGHT
The energy that an object has because of its height above the ground is known as its potential energy (PE). The following formula is used to compute it:

The formula is PE=m⋅g⋅h. 

Where:

  • m = mass of the object (in kilograms)
  • g = acceleration due to gravity (approximately 9.8 m/s² on Earth)
  • h = height above the ground (in meters)

Law of Conservation of Energy

The Law of Conservation of Energy states that energy cannot be created or destroyed; it can only be transformed from one form to another. In a closed system, the total amount of energy remains constant over time.

Rate of Doing Work (Power)

Power is defined as the rate at which work is done. It measures how quickly work is performed or energy is transferred over time.

Formula:

The formula for calculating power (P) is:

P=Wt

Where:

  • P = Power (in watts, W)
  • W = Work done (in joules, J)
  • t = Time taken (in seconds, s)
SI unit of Power is Watt (W)

Average Power = Total work done or total energy used/Total time taken

Commercial Unit of Energy 
  • For commercial purpose, bigger unit of energy is Kilotwatt hour (KWh).
  • 1 KWh: 1 KWh is the amount of energy consumed when an electric appliance having a power rating of 1 Kilowatt is used for 1 hour.