Download Conservation of Energy - Warren County Schools

Conservation of Energy
IT’S THE
LAW
Lifting a Ball



When you lift a ball to a
certain height you do
work on it.
This work (W) is equal
to the weight of the ball
(w=mg) times the
height (h).
The work done to lift
the ball is stored as
potential energy.
PE = W = mgh
h
W = Fd =wh = mgh
Lifting a Ball



The ball now has potential
energy stored in it.
This means the ball has the
potential to do work if you
drop it on something.
But before it can do that
work it must convert the
PE to another form of
energy called kinetic
energy – energy of motion
PE = mgh
h
Kinetic Energy
Energy of Motion
Kinetic Energy
Kinetic Energy (KE)
is energy of motion.
 Kinetic Energy
depends on the
moving object’s mass
(m) and the square of
its speed (v).

1
2
KE = mv
2
Lifting a Ball
As the ball falls:
 Its Potential Energy
decreases because its
height decreases.
 Its Kinetic Energy
increases because it speeds
up.
 It turns out the amount of
potential energy lost as the
ball falls is the same as the
amount of kinetic energy
the ball gains!!!!
PE = mgh
h
Kinetic Energy
Energy of Motion
The Law of Conservation of Energy
Energy cannot be created nor destroyed, it
can just be transformed from one form to
another, or transferred from one object to
another.
If we ignore any losses in energy to sound or
heat (two big sources) this means that PE +
KE will always remain constant.
The sum of all the potential energy and the
kinetic energy of a system is defined as its
mechanical energy.
In an isolated, closed system, if there
are no losses in energy in the form of
sound or heat, then the mechanical
energy of a system is conserved.
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
PE = (2)(10)(5)
it falls.
=100-J
Height
5-m
4-m
3-m
PE=mgh
100-J
KE
0-J
ME
100-J
Since the
ball isn’t
moving,
KE = 0-J!!!!
100-J
100-J
2-m
100-J
1-m
100-J
0-m
100-J
PE + KE =
100 + 0 =
100-J
Conservation of
Energy says ME will
always be 100-J the
entire time the ball
falls!!!!!
5-m
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
4-m
3-m
PE=mgh
100-J
80-J
KE
0-J
20-J
ME
PE = (2)(10)(4)
=80-J
100-J
100-J
KE = 100 – 80
= 20-J
100-J
4-m
2-m
100-J
1-m
100-J
100-J
0-m
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
ME
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
100-J
1-m
100-J
0-m
100-J
PE = (2)(10)(3)
=60-J
KE = 100 – 60
= 40-J
3-m
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
0-m
ME
PE = (2)(10)(2)
=40-J
100-J
100-J
KE = 100 – 40
= 60-J
2-m
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
0-m
20-J
80-J
ME
100-J
KE = 100 – 20
= 80-J
PE = (2)(10)(1)
=20-J
100-J
1-m
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
ME
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
20-J
80-J
100-J
0-m
0-J
100-J
100-J
KE = 100 –0
= 100-J
PE = (2)(10)(0)
=0-J
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
ME
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
20-J
80-J
100-J
0-m
0-J
100-J
100-J
Where does the ball have
its maximum speed?
Just before it hits the
ground --- WHERE
ITS KE IS
LARGEST
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
ME
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
20-J
80-J
100-J
0-m
0-J
100-J
100-J
If the ball bounces back
up, what height will it
reach?
Since it only has 100-J
available, it will only
reach its original
height of 5-m!!!!
A Falling Ball
A 2-kg ball falls from a height of 5-m. Trace its PE & KE as
it falls.
Height
5-m
PE=mgh
100-J
KE
0-J
ME
100-J
4-m
80-J
20-J
100-J
3-m
60-J
40-J
100-J
2-m
40-J
60-J
100-J
1-m
20-J
80-J
100-J
0-m
0-J
100-J
100-J
If the ball loses 20-J to
heat (air resistance) and
sound, how high will it
bounce?
Since it only has 80-J
(100 – 20) available, it
will only reach a
height of 4-m!!!!
Pendulums & Roller Coasters



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Pendulums and Roller Coasters must follow
the Law of Conservation of Energy
IT’S THE LAW
This means that what ever potential energy is
stored in each at the beginning is transferred
back and forth between PE & KE.
If we are allowing for losses in energy from
heat and sound, then this amount of energy
slowly disappears.
A Pendulum
PE
KE
A Pendulum
If there are losses in energy to heat and sound, then the height it
reaches each swing gets lower and lower until it stops swinging.
Roller Coasters


Roller Coasters use the potential energy
stored when it is at the top of a hill and
converts it to kinetic energy throughout the
ride.
WEeeeeeeeee!!!!!!!!!