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Lesson Note

Subject: Basic Science And Technology

Class: JS2

Topic: The concept of work, energy and Power.

Learning Objectives: By the end of the Lesson, The learners should be able to:

1. Explain the concept of work, energy and power;

2. Use the formula: power= work done/time;

3. Explain the meaning of potential energy and kinetic energy;
4. State energy transfer that occurs when Work is done

5. State energy transfer that occurs when Work is doneLesson Summary Aids: See reference materials below contents.

## Lesson Summary / Discussion

Introduction
We are conversant with Work, energy and power as we often use them in our daily conversations. Work is believed literarily to mean any kind of physical and mental activities while power is expressed in terms of strength. However in science, these terms: work, energy and power have different meanings. For work to be done 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. You are familiar with the concept of energy as the ability to do work. But the new thing to be considered here about energy is that it is treated in relation to other aspects of our daily lives. In this lesson, we will focus on the concepts of work, energy and power as well as their calculations.

## 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 (f) x distance (d) moved in the direction of the force i.e work = (f x d). Work can simply be defined as the product of distance moved and the force applied in the direction of movement. The term work was introduced in 1826 by the French mathematician Gaspard-Gustave Coriolis as “weight lifted through a height”, which is based on the use of early steam engines to lift buckets of water out of flooded ore mines. 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 work and energy are measured in units called joules, named after the scientist P. Joules who carried out early studies on energy.

## Force

Force is a physical quantity that denotes ability to push, pull, twist or accelerate a body which is measured in a unit dimensioned in mass × distance/time².

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 i.e. F= m x a.
where F = force, m= mass and a = acceleration
The unit of force is Newton (N)
If force = mass x acceleration
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.

Example
What work is done when a mass of 5.00 kg is
raised through a vertical height of 2.5m (acceleration due to gravity is 10 m/s²)?

Solution

Work done = mass x acceleration x distance
W = 5.00kg x 10x 2.5m
= 125 joules

## 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 like the one given below:

Example
What is the power of a child that has done work of 70J in 10 seconds?

Solution
P = Work / Time = 70/10 = 7 Watts

## Energy And Its Sources

Energy has been defined as the capacity to do work.
The following are the various forms of energy:
1. Solar energy
2. Heat energy
3. Light energy
4. Mechanical energy: This is further divided into two: potential energy and kinetic energy.

5. Electrical energy
6. Chemical energy
7. Sound energy.

The main source of energy is the sun. It comes as light and heat energy and transformed to other forms. All forms of energy can be transformed or changed from one form to another. Electrical energy can be changed to light energy, such as when electricity passes through an electric bulb. Chemical energy can be changed to heat, e-g. when you light a kerosene lamp. The law of conservation of energy explains this transformatory behaviour of energy. It states that energy can neither be created nor destroyed but can be changed or transformed 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 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 towards 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 man running, a falling orange, a fired bullet, a rolling ball, etc possess kinetic energy.

## CALCULATION

### Calculating work, potential and kinetic energy, and power

Example 1
Calculate the work done if a box is pulled by a person with a force of 170N through a distance of 70m

Work done =force x distance
Force = 170N
Distance = 70m
Work done = 170 x 70 = 11900 joules

Example 2
Suppose a body of mass 1 kg is lifted through a height of lm, how much work is done.

The force of gravity on a mass of 1 kg is 10 newtons.

Distance moved by the force is 1m

Work done (force x distance = 10N x lm joules).

## Energy Transfer When Work Is Done

When an object is dropped from above the ground, work is done as the object is pulled to the ground. As the object is falling and work is done, the potential energy of the body is changed to kinetic energy. In principle, the quantity of potential energy stored in a body is always equal to the kinetic energy produced when the body is released to do work. In other words, when energy changes, for example, from potential to kinetic, there is always an accompanying work done.
Done studying? See previous lessons in Basic Science/Technology.

## Lesson Evaluation / Test

1. Define the following:
a. Work
b. Energy
c. Power
d. Force
2. Mention the forms of energy.
3. Which is the major source of energy?
4. What is the power of a child that has done work of 60J in 10 seconds?
5. Calculate the work done if a box is pulled by a person with a force of 150N through a distance of 50m.
6. Explain potential and kinetic energy.
7. What happens when an object is dropped from above the ground?
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## Resources

Image Credit: vectorstock.com

Coriolis, Gustave (1829). Calculation of the Effect of Machines, or Considerations on the Use of Engines and their Evaluation. Carilian-Goeury, Libraire (Paris). Retrieved 02-05-2022.

Definition of force Wikipedia.org, retrieved 02-05-2022.