Transcript Energy and its Conservation

Energy and its Conservation
Physics
Mrs. Coyle
Part I
• Mechanical Energy
– Potential
– Kinetic
• Work Energy Theorem
Energy and Work
• Energy is the ability to do work.
• Work is the energy transferred to
or from a system by a force that
acts on it.
Video Link: Kinetic Sculpture
• http://www.youtube.com/watch?v=
WcR7U2tuNoY&feature=related
Energy
• Symbol: E
• Scalar
• Units:
– J, Joule
– cal, calorie
– kcal, kilocalorie (Cal)
Mechanical Energy
• Potential Energy
• Kinetic Energy
Kinetic Energy, KE= 1 mv2
2
Energy of Motion
Problem 1
A bird of mass 2kg is traveling at a speed of
6m/s. What is its kinetic energy? (A: 36J)
If the speed of the bird doubles what
happens to its kinetic energy?
Question
• Can Kinetic Energy be Negative?
Work-Energy Theorem
W=KE
W=KEf-KEi
The net work done on an object is
equal to the change in its kinetic
energy.
Problem 2
If Robin Hood applies a 15N
force to pull his bow back by
25cm, with what speed will
his 0.150kg arrow leave the
bow?
A: 7.1m/s
Problem 3
What is the
work done by
gravity as an
8 kg object
falls from rest
at 15m to
5m?
What is the
object’s
speed at 5m?
A: 800J, 14m/s
Work Done by the Gravitational
Force
System is an object and the earth.
As an object falls:
W= mgh
As an object is thrown up:
W= -mgh
Potential Energy, PE=mgh
• Energy of position
• Stored energy
• Symbol: U or PE
• Unit: Joule
• Compared to a
Reference Point
(base level)
Problem 4
A flower pot of mass
5kg is located 15m
above the ground.
A)What is its potential
energy with respect
to the ground?
B) What work was
done to raise it to its
position?
A: 750J
Part II
• Conservation of Energy
Conservation of Energy
• Energy can neither be created nor
destroyed. It can only change from
form to form.
• Mechanical Energy, PE and KE
• Conservation of Mechanical Energy
PE1 + KE1 = PE2 + KE2
What happens when friction is
present?
• When friction is present, the work
done by the frictional force W=fd is
transferred to heat energy.
• Friction is a “non-conservative” force.
A Force is “Non-Conservative”
if:
• “the work it does on an object that moves
between two points depends on the path
taken.”
• “the work it does on an object that moves
through a round trip is non-zero.”
• Example: friction, tension, normal force,
propulsion forces.
A Force is “Conservative” if:
• “ the work it does on an object that moves
between two points depends only on the
position of these two points and not on
the path.”
• “the work it does on an object that moves
through a round trip is zero.”
• Example: gravity, force of a spring.
Problem 5
A 3kg watermelon sits on
a table 1.2m above the
ground.
a) What is its PE at the top
compared to the ground?
b) What is its KE at the top?
c) With what speed will it hit
the ground?
A: 36J, 0J, 4.9m/s
Problem 6
A penny is at the top of
the Empire State
Building 381m above
the ground. It is then
released. With what
speed will it hit the
ground?
A: 87.3 m/s
Problem 7-Roller Coaster
A roller coaster car started from point A
at a height of 100m. What is its speed
at point B?
A: 44.7m/s
Problem 8 - Pendulum
a) What is the
potential energy of
the bob at point A
compared to the
ground?
b) What is the speed
of the bob at the
bottom?
A: 2.3J, 1.1m/s
A