# Lesson Notes By Weeks and Term - Senior Secondary 1

Work, energy, and power

Term: 1st Term

Week: 9

Class: Senior Secondary School 1

Age: 15 years

Duration: 40 minutes of 5 periods each

Date:

Subject:      Physics

Topic:-       Work, Energy and Power

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

1. Discuss the concept of Work, Energy and Power

INSTRUCTIONAL TECHNIQUES: Identification, explanation, questions and answers, demonstration, videos from source

INSTRUCTIONAL MATERIALS: Videos, loud speaker, textbook, pictures

INSTRUCTIONAL PROCEDURES

PERIOD 1-2

 PRESENTATION TEACHER’S ACTIVITY STUDENT’S ACTIVITY STEP 1 INTRODUCTION The teacher reviews the previous lesson on Pressure, Archimedes principles and upthrust Students pay attention STEP 2 EXPLANATION He discusses the concept of work and energy Students pay attention and participates STEP 3 DEMONSTRATION He discusses the concept of power Students pay attention and participate STEP 4 NOTE TAKING The teacher writes a summarized note on the board The students copy the note in their books

NOTE

WORK, ENERGY AND POWER

WORK

Work is defined as the product of force and distance in the direction of the force. It is a scalar quantity & measured in Joules.

Mathematically:

W = F X d                                       1.

W = mgh                                        2.

If a force is applied on a body at an angle Ø to the horizontal

Work done = FcosØ x d                                           3.

Work done to raise the body to an appreciable height

= FsinØ x d                                                               4.

Example

A boy of mass 50kg runs up a set of steps of total height 3.0m. Find the work done against gravity

Solution

m = 50kg, h = 3m, g = 10m/s2

Work done = m x g x h

= 50 x 10 x 3

= 1500 Joules

ENERGY

Energy is defined as the ability to do work. It is a scalar quantity &measured in Joules. There are many forms of energy. These include:

1. Mechanical energy
2. Thermal energy
3. Chemical energy
4. Electrical energy
5. Nuclear/Atomic Energy
6. Solar/Light energy
7. Sound Energy

POTENTIAL AND KINETIC ENERGY

Potential Energy - is simply “stored energy” i.e. energy possessed by a body by virtue of its states:

P.E = mgh                                       5.

Kinetic Energy: is the energy possessed by a body by virtue of its motion. Examples area student running a race, wind or air motion, electrical charges in motion, a moving bullet

K.E = ½ mv2                                                     6.

Example I

An object of mass 5kg is moving at a constant velocity of 15m/s.

Calculate its kinetic energy.

Solution

K.E = ½ mv2

= ½ x 5 x 15 x 15

= 562.5 J

Example II-

Find the potential energy of a boy of mass 10kg standing on a building floor 10m above the ground level. g = 10m/s2

Solution:

P.E =m x g x h

= 10 x 10 x10

=1000 J

POWER

Power is defined as the rate of doing work or the rate of transfer of energy. It is a scalar quantity & measured in watt

Power = work done

Time                                            7

P = (F X d)/t = F X d/t = FV                         8

Example -A boy of mass 10kg climbs up 10 steps each of height 0.2m in 20 seconds. Calculate the power of the boy.

Solution

Height climbed = 10 x 0.2 = 2m

Work done = m x g x h = 10 x 10 x 2= 200 Joules

Power = work

Time

=    10 x 10 x 2

20

= 10watts

EVALUATION:   1. Define these terms

(i) power

(ii) work done

(iii) energy

1. A boy of mass 960g climbs up to 12 steps each of height 20cm in 20 seconds. Calculate the power of the boy
2. Explain work done.

4(a) State the law of conservation of energy.

(b) A body is displaced through a certain distance x by a force of 30N. If the work done is 100J and the displacement is in the direction of force, what is the value of x?

1. A motor can converts chemical energy of petrol to mechanical energy at 30% efficiency. Calculate the mechanical energy obtained from 10litres of petrol. (1 litre of petrol contains 2.8kJ of chemical energy)
1. A student eats a dinner container 8.0x106J of energy. He wishes to do an equivalent amount of work in a nearby gym by lifting a 60kg object. How many times must he raise the object to expand this much energy? Assume that he raises it a distance of 2.0m each time

CLASSWORK: As in evaluation

CONCLUSION: The teacher commends the students positively