Transcript WORK and ENERGY

The Definition of Energy
Energy is the ability to conduct work. The unit of energy in
SI is joule (J). The other unit for energy is calorie (cal). The
relation between joule and calories is as follows.
1 calorie = 4.2 joule or
1 joule = 0.24 calorie
Forms of Energy
a. Heat energy
b. Electric energy
c. Sound energy
d. Light energy
e. Nuclear energy
The use of energy in daily life
a. Solar energy
b. Geothermal Energy
c. Wind energy
d. Tidal energy
e. Water energy
f. Nuclear energy

Non renewable

Renewable
› Oil (petroleum)
› Solar
› Natural gas
› Wind
› Coal
› Geothermal
› Uranium (nuclear)
› Biomass
› Hydro
› Ocean
Energy can be transformed into many
forms, such as :
 Light to heat
 Electric to light
 Electric to heat
 Electric to sound
 Chemical to electric
Energy can be transformed into many
forms, but the amount of energy is
constant. There is no increasing or
decreasing.
 Only the form of energy has changed,
not its amount.


Kinetic energy is the energy in the form
of movement, such as:
› The running children
› The rock fall
› The running car
m1
m2
m
m
m2 bigger than m1

Kinetic energy depend on the mass and
velocity of the moving object
› The larger the mass, the larger its kinetic
energy
› The thing move faster, its kinetic energy
become larger

We can write formula kinetic energy
KE = ½ m
2
v
› KE = kinetic energy (joule)
› m= mass of moving object (kg)
› V = velocity of moving object (m/s)

Unit of kinetic energy in SI system is:
› Joule
› Kg m2/s2
› N.m
› Watt. Second

Another unit is:
› Kalori
1 Kalori = 4.2 joule

A car have mass 50 kg, it move10 m/s.
Calculate a mount of kinetic energy!

Its amount depend on
› its the position or condition of the object
 If its position so higher, its potential energy is
greater
› Its mass
 The higher its mass, its potential energy is
greater

We can write formula gravitational
potential energy
PE = m g h
› PE = gravitational potential energy (joule)
› m = mass of object (kg)
› h = high (m)

Unit of potential energy in SI system is:
› Joule
› Kg m2/s2
› N.m
› Watt. Second

Another unit is:
› Kalori
Example
Example
Exercise :
1. If the acceleration due to gravity in a certain place is 10 m/s2, then
calculate the potential energy of a body, which mass is :
a. 4 kg at the height of 25 m above the ground
b. 1.5 x 10-5 gram at the height of 6 x 106 cm
2. Calculate the kinetic energy owned by :
a. car of 2500 kg which moves velocity of 72 km/hour
b. a ball 80 gram which moves with velocity of 15 m/s
3. A car is pushed by 3 people. They are Iman with pushing force of 300 N,
Ferry of 500 N, and Dias with pushing force of 600 N. The car moves as far
as 200 m from the initial place.
a. What is work of each people ?
b. What is total force that moves the car ?
c. What is total work of the three people ?
What is the potential energy if the rock
that have mass 2 kg stay on the table
that have high 1.5 m.
 The water have potential energy 36,000
joule, stay on the mountain that have
high 500 m, calculate amount of water.


Look at the picture





See in the picture
When its goes down, its potential energy
change to kinetic energy
At the slowest point, its kinetic energy is
greatest and potential energy is smallest.
When it goes up, the kinetic energy
changes to potential energy.
At the highest point, its potential energy is
greatest
From example:
 The potential energy is changed from
kinetic energy continuously.
 Potential and kinetic energy together in
form mechanical energy.
 It is a sum of potential and kinetic energy
in a system


Formula of mechanical energy
Em = Ek + Ep
Em = mechanical energy
 Ep = Potential energy
 Ek = kinetic energy

Energy can be transformed in one form
to another, but its total energy is
constant. In other word, energy is
conserved.
 That is called the law of energy
conservation
 Em1 = Em2 = Em3= Constant

Highest point
Mechanical energy
is in the form of
potential energy
lowest point
Mechanical energy
is in the form of
kinetic energy.
h
ground
Em = Ep
Em = Ep + Ek
Em = + Ek
Em = Ep
Em = Ep + Ek
Em = + Ek
Calculate the K.E of
a. 1 kg trolley traveling at 2 m/s
b. 2 g bullet traveling at 400 m/s
2. What is the velocity of an object of mass 1
kg has 200 j of K.E ?
1.
WORK and ENERGY
Work
W=Fxd
Helped by
Simple
machines
examples
Mathematically
formula
The law of
Conservation
Of energy
fulfill
needs
WORK
ENERGY
examples
Gravitational
Potential energy
PE = m.g.h
Conservation
of mechanical
Energy
EMinitial = EMfinal
Consists of
Kinetic energy
KE = ½ m.v2
•Lever
•Pulley
•Ramp (incline
plane)
•Gear
• Solar energy
• Electric energy
• Nuclear energy
• Geothermal
energy
• Mechanic
energy
Exercise
1. What is the work needed to push the table with force of 50 N as far as 2
m?
2. How much work you must do to lift an object with a mass of 1.5 kg for 2
m high if the gravitational acceleration in that place is 10 m/s2 ?
3. A car moves as far as 0.5 km with work 3,750 joule. What is the
magnitude force of the car ?
4. A body is placed 10 m high from the ground and has a mass of 10 kg. If
the acceleration due to gravity is 9.8 m/s2, what is the potential energy
that body has ?
5. A car with a mass of 800 kg moves with velocity of 10 m/s. what is the
kinetic energy of the car ?
A car of mass 1200 kg is moving a
constants speed of 72 km/h
a. find the speed of car in m/s
b. calculate the kinetic energy of car ?
2. A bullet of mass 20 g leaves a gun with
kinetic energy of 10 kJ. Calculate the
speed of bullet ?
1.