# Work, Energy and Power

SUBJECT : BASIC SCIENCE AND TECHNOLOGY

CLASS : JSS2

TERM : SECOND TERM

WEEK : WEEK 1

TOPIC :

ENERGY
1.Types of energy
a. Kinetic energy
b.Assumption

2.Explanation of some
phenomena using kinetic theory

BEHAVIOURAL OBJECTIVES : At the end of the lesson , pupils should be able to

• define energy
• mention types of energy
• explain some phenomena using kinetic theory

CONTENT

Week 1

Topic: Work, Energy and Power

Introduction

Work and energy are common concepts we always encounter in our lives on a daily basis. If one does not have energy, one cannot do any work. The stored up energy (potential energy) in our muscular system is converted to kinetic energy. When work and energy are related, they have the same unit of measurement called Joule.

Concepts of Work, Energy and Power

Work

Work is said to be done when a body moves in the direction of the force i.e.

Work = force x distance moved in the direction of the force

W = f x d

Where W = work done,

f = force (F = mass (m) x acceleration of free fall due to gravity (g))

g = 10m/s2

d = distance moved in the direction of the force

Therefore, a force of gravity acting on a 2kg box is 20N; and on a 3kg box is 30N etc.

It is important to note that if the force applied to a body cannot cause motion or displacement, work is not done. For example, if a man was pushed against a wall so much that he began to sweat, but the wall did not move, one cannot say that he has done any work.

Energy

Since work done is the product of force and distance in the direction of the force, therefore,

F = mg

W = mgh

Where m = mass in kg

g = Acceleration of free fall due to gravity in m/s2

h = distance moved by the force in metres

work done on a 2 kg box = 2 x 10 x 1.5

= 20N x 1.5m

= 30Nm

(Newton metres is the same as joule)

Work done on a 5kg box = 50N x 1.5m

=     75J

The amount of work done on a 5kg box is the greatest, therefore, the 5kg box requires the greatest amount of energy and the 2kg box requires less amount of muscular energy.

Therefore, work done is the measure of the energy used. Then, we can define energy as the ability to do work.

Potential energy is the energy possessed by a body by the virtue of its position and kinetic energy is the energy possessed by a body in motion. These two types of energy are generally called mechanical energy.

Potential energy (W)= mgh

Kinetic Energy (K.E) = ½mv2

Power

Power is defined as the rate of doing work.

P = Work done/ Time

P = W/t

Work done = Fs = mgs

Therefore, P = mgs/t

Where:

P = power in Watts

F = force in Newton

T = time in seconds

M = mass in Kg

g = acceleration of free fall due to gravity

The S.I. unit of power is Joules per second (J/s) or Watts (W)

Calculation Involving Work Done Per Time (Power)

Example: How much power does a student of 25kg mass who climbed a stair with 20 steps and one step is 15cm high in 30s has? (Assume g = 10m/s2).

Solution:

Distance covered = 20 steps x 15cm

= 300cm = 3m

Power = mgs/t

= 25kg x 10m/s2 x 3m / 30s

= 25 Watts

Energy Transfer: Conversion of Potential Energy to Kinetic Energy

Simple Pendulum

At point A and C, the potential energy is maximum and kinetic energy is zero (because there is no motion at those points). However, at point B the kinetic energy is maximum and potential energy is zero (because the motion is maximum at that point). We can therefore conclude that the potential energy on the bulb at point A has been converted to kinetic energy at point B and the kinetic energy of the bulb at point B has been converted to potential energy at point C.

Furthermore:

Work, Energy and Power

• Meaning or work, energy and power
• Concept of work, energy and power
• Forms of energy (heat, light, kinetic, potential etc.)

Meaning of work, energy and power

Work, energy and power are often used in every day conversation. Work is thought to mean any kind of physical and mental activities, while power is expressed in terms of strength. In science, these terms: work, energy and power have special meanings. For work to be used in science, two things are necessary. There will be force and the force must produce motion. Power on the other hand is the rate at which work is done. Energy is the ability to do work, however the new thing to consider here is that it is considered in relation to other aspects of our daily lives. In this chapter, the concept of work, energy, power and their calculation will be explained.

Concept of work, energy and power

Work:

Work is a product of force and distance moved in a given direction, and the quantity of work done is always equal to the quantity of energy put in. In science, work is said to be done when a force can produce movement in a measured direction, i.e. work = force X distance (f X d). Work can simply be defined as the product of distance moved and the force applied in the direction of movement. Note that the useful force is the part of the force, which acts in the direction of movement. If the force is directed in another direction other than that of motion, its component in the direction of motion is the one to use to multiply the distance to obtain the work done.

Generally, for any work done, there must be energy input since energy is the capacity of any system or a body to do work. Both energy and work are measured in units called joules, named after the scientist P. Joules who carried out earlier studies on energy.

Force is that which changes a body’s state of rest or uniform motion in a straight line. It can as well be expressed as: Force = mass X acceleration (F = M X A) where F is force, m is mass and a is acceleration. The unit if force is Newton. If force = mass X action, then Work can be given as: work = mass X acceleration X distance.

This is a simple formula that can be used to calculate work done especially against gravity.

Work done and energy transferred are measured in joules (J). The work done on an object can be calculated if the force and distance moved are known.

A change in momentum happens when a force is applied to an object that is moving or is able to move. The total momentum in an explosion or collision stays the same.

Work, force and distance

You should know, and be able to use, the relationship between work done, force applied and distance moved.

Background

Work and energy are measured in the same unit, the joule (J). When an object is moved by a force, energy is transferred and work is done. But work is not a form of energy – it is one of the ways in which energy can be transferred.

The equation

This equation shows the relationship between work done, force applied and distance moved:

work done (joule, J) = force (newton, N) × distance (metre, m)

The distance involved is the distance moved in the direction of the applied force.

Power:

Power is also related to the concepts of energy and work. Power is defined as the rate of doing work, i.e. work done divided by time.

Power = Work done

Time taken

The unit of power is watt (w), you can use the formula to solve problems.

Example:

What is power of a child that has done work of 50J in 10 seconds?

Solution:

P = Work              =             50           =             5 watts

Time                               10

Forms of energy:

Energy has been defined as the capacity to do work. The following are the various forms of energy:

• Solar energy
• Heat energy
• Light energy
• Mechanical energy (this is further divided into two: potential energy and kinetic energy)
• Electrical energy
• Chemical energy
• Sound energy

The main source of energy is the sun, it comes as light and heat energy and transformed or changed to other forms. All forms of energy can be transformed or changed from one form to another to another. Electrical energy can be changed to light energy, such as when electricity passes through an electric bulb. Chemical energy can be changed into heat e.g. when you light up a kerosene lamp. The law of conservation of energy explains the transformatory behavior of energy. It states that energy can neither be created nor destroyed but can be changed from one form to another. All forms of energy are measured in Joules.

Potential and Kinetic energy

A stone on the ground does not have energy so long as it is lying on the ground, the stone cannot be seen doing any work. However, if a stone is placed on a table and if it falls off, it can break a lamp on which it falls. The stone here has done some work by virtue of its position. Therefore, when the stone is on the table, it has energy stored up as a result of its position. The type of energy possessed by a body due to its position is called Potential energy. This energy increases as the height of the table increases and it decreases as it falls to the ground. When it reaches the ground, it has zero potential energy. On the other hand, kinetic energy is the energy possessed by a moving body. For example, a moving car, a running man, a falling orange, a fired bullet, a rolling ball, etc. possess kinetic energy.

An object can store energy as the result of its position. For example, the heavy ball of a demolition machine is storing energy when it is held at an elevated position. This stored energy of position is referred to as potential energy. Similarly, a drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow. Yet when its position is altered from its usual equilibrium position, the bow is able to store energy by virtue of its position. This stored energy of position is referred to as potential energy. Potential energy is the stored energy of position possessed by an object.

PRESENTATION

The lesson is presented step by step

Step 1 : Revision pf the previous topics

Step 2 : Introduction of the new topic

Step 3 : Allow the learners to give their own contributions by asking questions ,giving suggestions or making contributions and correcting them when the need arises

CLASS ACTIVITIES

Teacher; Guides
students to carry out activities on boiling and evaporation and discuss
findings.

Students: Deduce the
factors that affect evaporation from their observation.

EVALUATION

1. What is energy
2. What is power
3. What is kinetic energy
4. What is potential energy
5. How much power does a student of 25kg mass who climbed a stair with 20 steps and one step is 15cm high in 30s has? (Assume g = 10m/s2)?

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