PHYSICS S.S.S ONE (ENERGY AND POWER) 1ST TERM
FIRST TERM: WEEK 6
TOPIC: -
WORK, ENERGY AND POWER
INSTRUCTIONAL MATERIALS: - Table, ball
REFERENCES BOOK: -
1.
O. E. FARINDE e tal, Essential
physics FOR SENIOR SECONDARY SCHOOLS,
Tonads publishers.
2. M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR
SENIOR SECONDARY SCHOOLS, Africana first publishers.
Olatunbosun
K. (2004), CALCULATION IN PHYSICS FOR
SSS,
PREVIOUS KNOWLEDGE: - Students have been doing one work or the
other daily at home.
OBJECTIVES: - At the
end of the lesson students should be able to: -
1. Define
work.
2.
Mention forms of energy.
3.
Differentiates between kinetic and potential energy.
4.
Solve mathematical problem on work and energy.
5.
Define power.
6.
The SI unit of work, energy and power.
CONTENT:
-
WORK,
ENERGY AND POWER.
Concept Work
Work is done when a force is exerted to move an object through a distance in
the direction of the force.
Work, W, is defined as product of the force and the displacement of an object
in the direction of the force.
Work=Fs
Where, F= the force acting, S= the displacement (or distance traveled in
the direction of the force)
Work is a scalar quantity and its unit is joule (J) or N m. 1 joule =1Nm
A force applied at an angle θ to the horizontal, the component
of the force is given by F cos θ
d
= horizontal distance
so, work = F x d = Fcos θ x d
Work done to lift a body / work done against gravity
Since weight of a body act downward the work done if a body is
lifting up i.e. going against weight is analysed below.
The work done = force x
distance
=
mg x h. where g = acceleration due to gravity in ms-2
Work done to lift up against weight of the body = mgh. M = mass
of the body, h= vertical height reach
not slanting height.
Work done on falling body
For a body to fall, force field (weight) does not work on the
body but the work done by gravity on the body is = mgh.
mg
Example:
1. A block which is at rest is acted on by force of magnitude 3 N in
different direction. Determine the work done by the block if the object move
for 2 m.
Solution
F=3 N; s= 2m.
Work done, FS = 3 x 2 = 6Nm
1 joule is the work done when a force of 1 Newton moves of an object for 1
m in the direction of the force .
Concept of Energy
Energy is the ability to perform work. Energy can neither be created nor
destroyed. It can only be transformed from one kind to another. The unit of
Energy is same as of Work i.e. Joules.
Forms of energy: viz:-Mechanical energy; Mechanical wave energy;
Chemical energy ; Electric energy; Magnetic
energy; Radiant energy; Nuclear energy; Ionization energy; Elastic energy; Gravitational
energy; Thermal energy; Heat Energy.
Types of Mechanical Energy
Mechanical energy is classified into two types: i. Kinetic
Energy ii. Potential Energy.
The energy in motion is known as Kinetic Energy ; K.E = ½ mv2
Potential Energy is the energy stored in an object ; P.E = mgh (
elastic potential = ½ Kx2)
CONSERVATION LAW OF MECHANICAL ENERGY
It states that although energy can be changed from one form to
another, the total energy of a given closed system remain unchanged i.e. energy
can neither be created nor destroyed during a transformation.
This application to K.E transformed to P.E as illustrated below.
The
body at rest position ‘at the top’ posses total energy which is P.E = mgh, K.E = 0. Since the body has no
velocity but have height ‘h’.
‘At
the middle’, the total energy is P.E + K.E, but ‘at the bottom’ the P.E is
changed to K.E = ½ mv2, P.E = 0
To
find the maximum velocity (Vmax), i.e total energy is always conserved i.e.
K.E=P.E
Example
2. A student of mass 50 kg walks up a flight of stairs 1.5 m high. What is the
work done by the student?
Work = Fx s
=mg x s =(50 x 10) N x 1.5 m =750 J
3. A garage hoist lifts a truck up 2 meters above the ground in
15 seconds. Find the power delivered to the truck. [Given: 1000 kg as the mass
of the truck]
First we need to calculate the work done, which requires the
force necessary to lift the truck against gravity:
F = mg = 1000 x 9.81 = 9810 N.
W = Fd = 9810N x 2m = 19620 Nm = 19620 J.
The power is P = W/t = 19620J / 15s = 1308 J/s = 1308 W. P=f.v
4. An object start to sliding from a point of A without the
initial velocity. If there is no friction force, what is the velocity of the
object at the lowest point.
solution
Mass of object = m ; Initial velocity (vo) = 0; Height (h) = 20 meters; Acceleration due to gravity (g)
= 10 m/; Final velocity (vt)= ?
Initial mechanical energy (ME1)
= Gravitational potential energy at point A (PEA) = m g h =
(m)(10)(20) = 200 m
Final mechanical energy (ME2)
= kinetic energy (KE) = ½ m vt2
The velocity of the object at
the lowest point (vt) ?
Apply the principle of
conservation of mechanical of energy states that the initial mechanical energy
= the final mechanical energy.
EM1 = EM2
200 m = ½ m vt2; 200 = ½ vt2; 400 = vt2; vt = 20 m/s
5. A 2-kg ball free fall
from point A, as shown in figure below (g = 10 ms-2). After arrive
at point B, the kinetic energy = 2 times the potential energy. What is the
height of point B above the surface of earth.
Known :
Mass of ball (m) = 2 kg; Acceleration
due to gravity (g) = 10 ms-2; Height of point A (hA)
= 90 meter; Height of point B (hB) = ?
Solution :
When arriving at point B, the kinetic energy of ball at point B =
2 times gravitational potential energy at point B.
EK = 2 EP
½ m v2 = 2 m g hB; ½ v2 = 2 g hB; v2 = 2(2)(10) hB; v2 = 40 hB
Velocity (v) of ball when arrive at point B after free fall from
point A :
v2 = 2 g h = 2(10)(90–hB) = 20(90–hB)
Substitute v2 at above equation with v2 at
this equation.
v2 = 40 hB; 20(90–hB) = 40 hB; 1800–20 hB = 40
hB; 1800 = 40 hB + 20 hB; 1800 = 60 hB; hB = 1800 / 60hB =
30m.
Concept
of Power
Power
is a physical concept that has several different meanings, depending on the
context and the information that is available. We can define power is the rate
of doing work. It is the amount of energy consumed per unit time.
Power,
P =
Unit
of Power
As
power don’t have any direction, it is a scalar quantity. The SI unit of power
is Joules per Second (J/s), which is termed as Watt. Watt can be defined as the
power taken to do one joule of work in one second. The unit Watt is dedicated
in honour of So James Watt, the developer of the steam engine.
Its other
units are kilowatt and horse power,
1
kilowatt = 1000 watt
1 horse
power = 746 watt
Example
6. Calculate the power of a body of mass 50kg
moving at a velocity of 2ms-1. (g = 10ms-2)
Solution
M= 50g, g= 10ms-2;
v= 2ms-1
Power = force X
velocity = mg X v = 50kg X 2ms-1 X 10ms-2 = 1000w =1kw.
PRESENTATION
Step I: The teacher
introduces the new topic to the students
Step II: The teacher
explains the concept of work
Step III: The
teacher leads the students to solve problems on work
Step IV: The
students define energy and mention forms of energy
Step V: The teacher
explains conservation law of mechanical energy with calculation involved.
Step VI: The teacher
defines power and leads the students in solving mathematical problem involved.
Step VII: The
teacher allows the students to ask question
EVALUATION
The teacher evaluates
the lessons by asking the following questions:
1.
Define work.
2.
Mention forms of energy.
3.
Differentiates between kinetic and potential energy.
4.
Solve mathematical problem on work and energy.
5.
Define power.
6.
The SI unit of work, energy and power.
ASSIGNMENT
Write short note on
renewable and non-renewable energy resources
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