Chapter 4 Work Energy Power

CHAPTER 4

Work, Energy & Power

Learning outcomes

 the students will be able to:

·        1. Define work, energy and power

·         2.Calculate the work in the form of kinetic and potential energy

·         3.Recognize the principal of conservation of energy.

·         4.Describe the conversion of energy from one to another.

·         5.Apply the concepts and formulas of work, power and energy in solving related problems.

·         6.Compute the efficiency by the mechanical system

“ Never mistake knowledge for wisdom. One helps you make a living, the other helps you make life ” 

                                               

*Sandra Carey*

INTRODUCTION

An apple falls of a tree and free falls to the ground. This is an example of work. It is because there is a gravitational force acts on the apple which causes the apple to be displaced in a downward direction. In order to do work, the force (e.g. force due to gravity) applied to an object must be parallel to the direction of the object. Work cannot be done without energy. Energy is defined as the ability of doing work.

Principal of conservation of energy states that energy can neither be created nor destroyed but it can be changed into other forms. Mechanical energy comes in two forms which are kinetic and potential energy. These two forms of energy involve in conservation of energy.

In science and engineering, most machines are designed to do work on objects for making humans’ lives easier. For example a car engine is described by power rating as how rapidly the car can accelerate. More powerful car engine means the car can accelerate in a short time. Some of the energy and work is lost (e.g. heat and vibration) during the process of accelerating. Thus, efficiency of the car machine is described as the measure of how much energy and work is conserved.

WORK

Definition

Work is done when force acts on a body and the body moves in the direction of an applied force. Work is measured by the product of force and displacement in the direction of force.

If the movement of the object does not follow the direction of the force, work done can be calculated using the formula at below.

W = F cos θ • s

Work is Zero

Work can be zero even with the presence of Force, F or displacement, s in certain conditions as below:

      a)      The force, F and displacement, s are orthogonal, θ = 90˚    

  

ENERGY

Energy is one of the most important concepts in science. In physics, energy is defined as the capacity of doing work. Traditionally, energy is defined as the ability to do work.

This simple definition is not very precise but it is still valid.

Energy is scalar quantity. The SI unit for energy is N m or Joule, J.

Forms of Energy

In everyday life, energy exists in many forms :

·         Mechanical (potential and kinetic)

·         Chemical

·         Nuclear

·         Thermal or heat

·         Light

·         Electrical

·         Sound

Types of mechanical energy

Kinetic Energy

An object in motion has the ability to do work and can be said to have energy.  The energy of motion is called as kinetic energy, from Greek word kinetikos, meaning motion. A moving object can do work on another object it strikes. For examples, a moving hammer does work on a nail it drives into wood and a flying cannonball does work on a brick wall it knocks down. In both cases, the moving object exerts a force on second object which undergoes a displacement.

  

  Potential Energy

An object can store energy as the results for its position. Potential energy can be defined as the energy associated with forces that depend on its position of an object relative to its surrounding. It can also be defined as the energy possessed by an object due to its position or state.

Gravitational potential energy

Gravitational potential energy is the most common example of potential energy and it is depends on the vertical height of the object above some reference level. In order words, potential energy due to gravity is the product of the object’s weight, mg and height, h above some reference level.

Gravitational potential energy is defined as energy possessed by an object due to its position in a gravitational field relative to some reference point. 

Potential Energy	Kinetic Energy
Dormant volcano	Active volcano
Stopped heart	Beating Heart
Standing on the diving board	Jumping off the diving board
A race car at the starting line	A race car speeding around a corner
A balloon with air	Air coming out of the balloon

CONSERVATION OF ENERGY

Principal of conservation of energy is defined as energy can neither be created nor destroyed, but it can change from one form to another.

POWER

In science and engineering, most machines are designed to do work on objects for making humans’ lives easier like transportations. For example, cars have their own mechanical system. A car engine is described by power rating as how rapidly the car can accelerate. More powerful car engine means the car can accelerate in a short time. Power is a measure of how quickly work is done. Power is defined as the rate of change of work done.

Power can also be defined as the rate of change of energy.

Power is a vector quantity and the SI unit is J s^-1 or Watt, W

EFFICIENCY

Efficiency is defined as a ratio of the amount of energy produced (energy output) to the energy used, expressed in percentages. In relation to car engine, some of the energy and work is lost (e.g. heat and vibration) during the process of accelerating. In other words, efficiency of the car machine is described as the measure of how much energy and work is conserved during the process in percentage. A perfect process would have hundred percent of efficiency.

Efficiency can also be defined as the measure of how much energy or work is conserved during the process in percentage. The symbol of efficiency is η (pronounced as eta).