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Energy
Chapter Overview
Energy drives every process in the universe. In this chapter, we will define Work and Energy, explore the concept of Mechanical Energy (Kinetic and Potential), and discuss the fundamental Law of Conservation of Energy.
3.1 Work and Energy
What is Work?
In Physics, work is said to be done only when a force applied on a body moves that body in the direction of the applied force.
- If you push a heavy solid wall all day and it does not move, absolutely zero work is done in physics, despite you feeling physically tired.
- If you push a light chair and it moves forward, work is officially done.
Energy: It is defined as the capacity or ability of a physical body to do work. Without energy, no work can be performed.
- The S.I. unit of both Work and Energy is the Joule ($J$).
3.2 Mechanical Energy
The total energy possessed by an object due to its structural position or its active motion is classified broadly as Mechanical Energy. Mechanical Energy is divided into two primary types:
1. Potential Energy (P.E.)
This is the stored energy possessed by a body due to its specific raised position or distorted configuration (shape).
- Gravitational Potential Energy: A heavy rock lifted to the top of an enormous cliff has stored energy because of its height. If you drop it, the stored energy does work by breaking the ground below.
- Elastic Potential Energy: A highly stretched rubber band or a compressed toy spring has stored energy because its shape was distorted. Taking your hand away releases this energy.
2. Kinetic Energy (K.E.)
This is the active energy possessed by a body strictly by virtue of its physical motion.
- A fast-moving cricket ball possesses high kinetic energy, which does work by crashing into and knocking off the heavy wooden stumps.
- Flowing river water has kinetic energy used to strongly rotate large heavy turbines in a hydroelectric dam.
AI Image Prompt: A vibrant, clear illustration of an archer holding a drawn bow. The string is pulled back incredibly tight (label: Elastic Potential Energy stored). An arrow is just released and is flying at high speed through the air (label: High Kinetic Energy in motion).
3.3 Law of Conservation of Energy
This is one of the most fundamental operational laws governing the entire universe.
The Law States:
Energy can neither be created out of nothing, nor can it be permanently destroyed. It can only be transformed or converted from one form to another.
As a result, the total energy of an isolated complete system remains absolutely constant.
Example: Falling Object
Imagine a heavy stone held firmly at the top edge of a high cliff. At this maximum height, it only possesses Potential Energy. Its kinetic energy is zero because it is not moving.
The exact moment you drop the stone, its height starts decreasing, so its potential energy decreases. However, its falling speed rapidly increases, meaning its Kinetic Energy increases. The lost potential energy is smoothly converted directly into kinetic energy. Right before it powerfully strikes the ground, its potential energy is zero, and its kinetic energy is maximum.
Q1. State the type of energy possessed by water stored behind a giant dam.
Ans: Gravitational Potential Energy.
Q2. What is the standard SI unit of Energy?
Ans: Joule ($J$).