Short Notes 1 – Chapter 10 – Work and Energy – Ncert- Class 9 – Science

Short Notes 1 –  Chapter 10 – Work and Energy – Ncert- Class 9 – Science

Chapter 10 – Work and Energy

10.1 Work

What is work? There is a difference in the way we use the term ‘work’ in day- to-day life and the way we use it in science. To make this point clear let us consider a few examples.

10.1.1 NOT MUCH ‘WORK’ IN SPITE OF WORKING HARD!

Scientific Definition of Work: Work W is calculated using the formula:

W=F⋅d⋅cos(θ)

Where:F is the force applied on the object,

d is the displacement of the object,

θ is the angle between the force vector and the displacement vector.

Examples of Work: In scientific terms, work is done when:

Lifting a book off the floor, because you exert a force upward and the book moves upward.

Pushing a box across the floor, because you exert a force in the direction of motion.

Carrying a load while walking, because you are exerting a force to overcome gravity and move the load horizontally.

No Work Done: In contrast, certain actions may seem like work in everyday language but don’t involve work in the scientific sense:

Holding a heavy object stationary above the ground, because there’s no displacement.

Carrying a load while standing still, because there’s no displacement.

Pushing against a wall, because there’s no displacement.

Understanding Work: It’s essential to recognize that in physics, work is not just about effort or energy expended. It specifically refers to the transfer of energy resulting from a force causing a displacement

10.1.3 WORK DONE BY A CONSTANT FORCE

Definition of Work: Work done by a force on an object is defined as the product of the magnitude of the force and the distance moved by the object in the direction of the force. Mathematically,

W=F×s, where

W is the work done,

F is the force applied, and

s is the displacement of the object in the direction of the force.

Unit of Work: The unit of work is the joule (J), which is equivalent to one newton-meter (N m). One joule of work is done when a force of one newton displaces an object by one meter in the direction of the force.

Sign Convention:

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

When the force and displacement are in opposite directions, the work done by the force is negative.

If the force applied causes the object to speed up or move in the direction of the force, the work is positive.

If the force applied causes the object to slow down or move opposite to the force, the work is negative.

Zero Work: If the force acting on the object is zero, no work is done, regardless of the displacement. Similarly, if the displacement of the object is zero, no work is done, regardless of the force applied.

Application of Force: The work done by a force depends on both the magnitude and direction of the force relative to the displacement of the object. Positive work is done when the force aids the displacement, while negative work is done when the force opposes the displacement.

Activity: An activity involving lifting an object illustrates the concept of positive and negative work. When lifting an object, the force exerted by the person is in the direction of the displacement, resulting in positive work. However, the force of gravity acts in the opposite direction of the displacement, resulting in negative work.

10.2 Energy

Sources of Energy:

The Sun is a significant natural source of energy for Earth.

Many energy sources, including solar, nuclear, geothermal, and tidal energy, are derived from various natural processes.

Other sources of energy might include wind, biomass, hydroelectric, and fossil fuels.

Definition of Energy:

In science, energy is defined as the capacity to do work.

It is a precise concept with specific implications for physical processes.

Objects with energy can exert forces on other objects, resulting in the transfer of energy.

Examples of Energy:

A fast-moving cricket ball hitting a stationary wicket demonstrates the transfer of energy, resulting in the wicket being thrown away.

An object raised to a certain height gains potential energy, which can be converted into kinetic energy when the object falls.

When a hammer falls on a nail, the energy from the hammer drives the nail into the wood.

Winding a toy car stores energy in the form of tension in a spring, which is converted into kinetic energy when the car is released.

Pressing a balloon changes its shape, demonstrating the potential energy stored in the compressed air.

Capacity to Do Work:

An object possessing energy has the ability to exert a force on another object and transfer energy to it.

The transferred energy enables the second object to move and perform work.

Measurement of Energy:

Energy is measured in the same units as work, which is the joule (J).

One joule of energy is the amount required to do one joule of work.

Larger units, such as the kilojoule (kJ), are sometimes used, where 1 kJ equals 1000 J.

10.2.1 FORMS OF ENERGY

Mechanical Energy:

Mechanical energy is the sum of potential energy and kinetic energy.

Potential energy is the energy stored in an object due to its position or condition.

Kinetic energy is the energy possessed by an object due to its motion.

Heat Energy:

Heat energy, also known as thermal energy, is the energy associated with the internal motion of particles in a substance.

It is transferred between objects as a result of temperature differences and can be converted into other forms of energy.

Chemical Energy:

Chemical energy is stored in the bonds between atoms and molecules within chemical substances.

It is released or absorbed during chemical reactions, such as combustion, metabolism, or chemical synthesis.

Electrical Energy:

Electrical energy is the energy associated with the movement of electric charges.

It is generated by the flow of electrons through conductive materials, such as wires, and is used to power various electrical devices.

Light Energy:

Light energy, or radiant energy, is electromagnetic radiation that is visible to the human eye.

It is emitted or absorbed by objects and can be converted into other forms of energy, such as chemical or electrical energy.

10.2.2 KINETIC ENERGY

Definition:

Kinetic energy is the energy possessed by an object due to its motion.

Objects in motion, such as a moving bullet, a rolling stone, or a flying aircraft, possess kinetic energy.

Relationship with Speed:

The kinetic energy of an object increases with its speed.

A faster-moving object can do more work than a slower-moving object.

Equation:

The kinetic energy (E) of an object with mass (m) moving at a velocity (v) is calculated using the formula:

E= ½ mv²

This equation relates the kinetic energy of an object to its mass and velocity.

Work-Energy Theorem:

The work done (W) on an object is equal to the change in its kinetic energy.

If an object starts from a stationary position (initial velocity, u = 0), then the work done is equal to ½ mv²  , representing the kinetic energy gained by the object.

Formula for an object moving with uniform acceleration:

s: Displacement of the object.

V² : Final velocity of the object, squared.

U² : Initial velocity of the object, squared.

a: Acceleration of the object.

The equation states that the displacement (s) is equal to the change in velocity (V² – U² ) divided by twice the acceleration (2a). This equation is derived from one of the equations of motion:

V^{2 =} U² +2as

By rearranging terms, we can derive the equation for displacement s:

  s = V² – U² / 2a

Now, substituting F=ma into the expression for work done (W), we get:

W=F×s=m×a× (V² – U² / 2a)

W= ½ ​m (V² – U² )

This equation represents the work done (W) by the force (F) in changing the kinetic energy of the object from u to v. If the object starts from rest (u=0), the equation simplifies to:

W= ½ m V²