Download 11 ENERGY ALL

FOR EVERYTHING THAT MOVES, THE MECHANICAL
ENERGY (PE + KE) IS CONVERTED TO HEAT BY
FRICTION. AFTER SOME TIME, ALL OF THE
MECHANICAL ENERGY IS CONVERTED INTO THERMAL
ENERGY AND ALL MOTION STOPS
NUCLEAR FUSION – TWO HYDROGEN MOLEUCLES
FUSE TOGETHER TO MAKE ONE HELIUM ATOM
THE CALORIE IS USED TO MEASURE HOW MUCH
ENERGY IS IN THE FOOD WE EAT. DIFFERENT TYPES
OF FOOD HAVE DIFFERENT AMOUNTS OF CALORIES.
EVERY GRAM OF FAT HAS 9 CALORIES. EVERY GRAM
OF CARBOHYDRATES AND PROTIEN HAS ONLY 4
CALORIES. NO WONDER EATING A LOT OF FATTY
FOODS WILL CAUSE US TO BE OVERWEIGHT.
the Work-Energy Theorem
If I do work on an object, the energy of
the object will change.
The change in the object’s energy is
equal to the amount of work I did on it.
the Work-Energy Theorem
If I do work on an object, the energy of
the object will change.
If I lift a ball up and put it on a shelf, now
the ball’s GPE is the same as the work I
did as I lifted it up.
the Work-Energy Theorem
If I lift a ball up and put it on a shelf, now
the ball’s GPE is the same as the work I
did as I lifted it up.
The change in the object’s energy is
equal to the amount of work I did on it.
KINETIC ENERGY IS THE ENERGY OF
THE MOTION OF AN OBJECT..
KINETIC ENERGY = (MASS) x (VELOCITY)2 / 2
KE = m x
2
v
/2
KINETIC ENERGY IS THE ENERGY OF THE MOTION OF
AN OBJECT..
KINETIC ENERGY = (MASS) x (VELOCITY)2 / 2
WORK = (FORCE) (DISTANCE)
IF I DO WORK ON AN OBJECT, THE CHANGE IN THE
OBJECT’S KINETIC ENERGY WILL EQUAL THE
AMOUNT OF WORK I DID.
IF I DO WORK ON AN OBJECT, THE CHANGE IN THE
OBJECT’S KINETIC ENERGY WILL EQUAL THE
AMOUNT OF WORK I DID.
For example – If I push on a book and increase it’s
velocity from zero to 2 m/s, I increased the Kinetic
Energy of the book.
The work I did on the book is equal to the increase
in it’s Kinetic Energy.
IF AN OBJECT INITIALLY HAS 20 JOULES OF KINETIC
ENERGY AND, AFTER YOU DO WORK ON THE
OBJECT, IT THEN HAS 60 JOULES OF KINETIC ENERGY,
THEN THE AMOUNT OF WORK YOU DID IS…
60 JOULES - 20 JOULES = 40 JOULES OF WORK
W = KE2 – KE1
W = KE2 – KE1
Work = Force x Distance
2
KE = mass x (velocity) / 2
So
F x d = (m x
2
v
/ 2)2 - (m x
2
v
/ 2)1
Gravitational Potential Energy
An object that has been lifted up against the
force of gravity has stored energy.
This stored energy is called
Gravitational Potential Energy
GPE = (MASS) (9.8) (HEIGHT)
Ug = m x g x h
Choosing a reference level
The choice of a reference level, at which the
potential energy is defined to be zero, is arbitrary.
The reference level may be taken as any
position that is convenient for solving a given
problem.
Conservation of Energy
The total amount of energy is constant. In
other words, energy can be neither created
nor destroyed.
Energy is conserved. The total amount of
energy remains the same.
Conservation of Energy
The total amount of energy remains the
same.
As an object falls, it loses height and thus
loses Potential Energy.
As an object falls, it goes faster and faster
and thus gains Kinetic Energy.
As it falls, the Potential Energy is changed
into Kinetic energy.
Conservation of Energy
The total amount of energy remains the
same.
When an object rises, just the reverse is
true…
As an object rises, the Kinetic Energy is
changed into Potential energy.
The total amount of energy remains the
same.
As it falls, the Potential Energy is changed
into Kinetic energy.
An any time during the fall, if you add
together the PE + KE you will get the same
amount of Total Energy.
We call this total amount of energy –
Mechanical Energy.
The total amount of energy remains the
same.
We call this total amount of energy –
Mechanical Energy.
ME = KE + PE
And ME will always be the
same (constant) at any point
in the fall or rise of an object
Conservation of mechanical energy
We will be concerned only with kinetic and
gravitational potential energy while investigating
motion in this course.
The sum of these forms of energy is referred to
as mechanical energy.
In any given system, if no other forms of energy
are present…
Conservation of Energy
As long as the system under investigation is
closed so that objects do not move in and
out, and as long as the system is isolated
from external forces, then energy can only
change form.
The total amount of energy is constant. In
other words, energy can be neither created
nor destroyed.
In a closed, isolated system, energy is
conserved. The total amount of energy
remains the same.
The path an object follows in reaching the ground
does not affect the final kinetic energy of the object.
Ug= mgh
Ug =(10.0 N)(2.00m)=
20.0 J
Total Energy = 20J
Half way down...
Ug= (10.0 N)(1.00 m)= 10.0 J
E= K+ Ug,
so
K= E- Ug
K= 20.0 J- 10.0 J =10.0 J