Download 7TH CLASSES PHYSICS DAILY PLAN

8TH CLASSES
PHYSICS
DAILY PLAN
SUBJECT: Work-Energy-Power
GOALS:
DURATION:
IN PRACTICE:
PRESENTATION:
Work:
The work is done if an object undergoes a displacement under
the effect of a force. The magnitude of the work is the product
of the displacement and the force.
F
Energy due to motion. If something is in motion it has a kinetic
energy. It depends on the mass and velocity of the object.
V
K.E.= 1/2 . m . v2
Example : Find the kinetic energy of a 400-kg car moving with
a speed of 10 m/s.
Solution :
K.E. = 1/2 . m . v2 = 1/2 . 400 . 100 = 20000j
Work-Energy Theorem :
Work is change I n kinetic energy. If work is done on an object
its the amount of work.
W =  K.E.
X
W=F.X
Unit of Work : The unit of work is joule.
Newton x Meter = Joule ( J )
Example : A 20-N force pulls a wooden block for 6 m. What is
the work done by the force?
Solution :
F
Potential Energy :
Energy due to the position
Gravitational Potential Energy
If an object has a distance (or a height) from the earth, it has s a
potential energy. The amount of energy is determined by the
mass of the object, its height and the gravitational constant.
m
P=mgh
h
X= 6 m
W = F . X = 20 . 6 = 120 J
Power:
Power is the work done in a unit time.
Work
Power =
Time
P.E. = m . g . h
Conservation of mechanical energy
When an object falls under only the influence of gravity: (no
friction):
h is decreasing, v is increasing
v
1
The unit of power is watt.
OR
h
PE is decreasing, KE is increasing
1
Joule
v
= watt .
second
Example : A child pulls an object for 15 meters with a force of
20 N in 4 seconds. Find the work done by the child and his
power.
Solution :
W = F . X = 20 . 15 = 300 J.
W
300
P=
=
= 75 Watt.
t
4
Energy :
Energy is the ability to do work. So its unit same with work.
There are many kinds of energy such as :
1. Mechanical Energy
2. Electrical Energy
3. Chemical Energy
4. Nuclear Energy
5. . . . . . . . . . . . . . ..
In this chapter we will only study the mechanical energy
There are two types of mechanical energy. Kinetic energy and
potential energy.
Kinetic Energy::
h2
2
Suppose the object falls from A to B.
A
g
Since the object is falling down,
potential energy decreases by (mgy).
y
h
Since gravitational force does work
kinetic energy increases by (mgy).
1
B
Therefore:
h
2
Increase in KE = Decrease in PE
KE= - PE
KE + PE = 0
(KEf – KEi) + (PEf – PEi) = 0
PEi + KEi = PEf + KEf.
If external work on an object is zero, total mechanical energy of
the object is conserved (=does not change):
MEi=MEf .
Ex:
PE = 100J
KE = 0
ME = 100 J
PE = 75J
KE = 25J
ME = 100 J
PE = 50J
KE = 50J
ME = 100 J
PE = 0
KE = 100J
ME = 100 J
{*The following 6 examples are to be solved by using
PEi + KEi = PEf + KEf method*}
Ex: An object of mass m is released from 5 m above the ground.
Find its velocity just before it touches the ground. {*Draw the
picture*}
HOMEWORK:
MULTIMEDIA: serway1,akadem,serway11
DEMONSTRATION:
EXPERIMENT:
TEACHER:
DIRECTOR: