Download What is Mechanical Energy?

Work and Energy
PowerPoint notes and practice
problems
When do we do “work”?
•
Two conditions must be met for work to be
done
1) The application of a force, and
2) The movement of something by that force in the
direction of the force applied
•
When we lift a load against Earth’s gravity
– The heavier the load or
– The higher we lift it
the more work we do
WORK
Work = Force x distance
W = Fd
Joule is the unit for work.
Something must be moving in order for
work to occur.
Calculating work
1. How much work is done if a box of
books is pushed 10 meters across the
floor with a force of 25 Newtons?
2. How much work is done if a barbell is
lifted 1 meter off the floor using a
force of 10 Newtons?
Calculating work
3. How much work is done if a box of
books with a mass of 50 kg is pushed
10 meters across the floor at 1 m/s?
• F = ma
• W = Fd
Calculating work
1. W = Fd
W = 25N x 10m = 250N·m or 250J
2. W = Fd
W = 10N x 1m = 10N·m or 10J
3. F = ma ; F = 50kg x 1m/s = 50N
W = Fd
W = 50N · 10m = 500N·m or 500J
Power
• Power is the rate at which work is done. It
equals the amount of work done divided by
the time interval during which the work is
done.
power = work done
time interval
power = W
t
The unit of power is joule per second, known as the
watt. One watt of power is used when one joule
of work is done in one second
Calculating work
4. If a box of books with a mass of 50 kg
is pushed 10 meters across the floor
at 1 m/s, how much power was used
to push the box?
• F = ma
• W = Fd
• P = W/t
Calculating work
4. F = ma; F = 50kg x 1 m/s = 50N
W = Fd; W = 50N x 10m = 500J
P = W/t = 500J /10s = 50J/s or 50W
ENERGY
• Fundamental building blocks of our
universe.
• Appears in different forms
• Has the ability to do work
• Any object that has energy has the ability to
create force.
• Units are measured, like work, in joules (J)
Mechanical Energy
• Mechanical Energy is the energy due to
the position of an object or the
movement of an object
• Mechanical Energy = Potential Energy +
Kinetic Energy
• ME = PE + KE
POTENTIAL ENERGY
• Comes from the position of an object
relative to earth.
• Stored energy
• Amount of potential energy must be the
same as the amount of work done to lift
the object against gravity.
• Objects do not use potential energy
until they move.
POTENTIAL ENERGY
• Usually referred to as gravitational
potential energy
gravitational potential energy = weight x height
Potential Energy Formula
9.8m/s2
PE = mgh
Potential energy
height
mass
In the last unit we learned that W = mg
PE = Wh
Potential energy
weight
height
Calculating Potential Energy
1. What is the PE of a boulder that weighs
100N sitting on a ledge 2m above the
ground?
2. What is the PE of a 4N can of paint at the
top of a 10 meter ladder?
3. If the limb of a tree has a PE of 90J in its
present position 15 meters above the
ground, what is it’s weight? What is the
limbs mass?
Calculating Potential Energy
1. PE = Wh
PE = 100N x 2m = 200J
2. PE = Wh
PE = 4N x 10m = 40J
3. PE = Wh
90J = W x 15m = 90J/15m = 6N
W = mg; 6N = m x 9.8m/s2 = .61 kg
KINETIC ENERGY
•
•
•
•
Energy in motion
Energy depends on Speed & Mass
Speed increases - KE increases
Mass increases - KE increases
Kinetic Energy Formula
• Kinetic energy = ½ mass x speed2
•
KE =
Kinetic energy
2
½mv
speed
mass
• When working this formula, always
square speed first.
Kinetic Energy Formula
• Kinetic energy is also equal to the net force
operating on an object and the distance it
travels or
Net force x distance = kinetic energy
Fd = kinetic energy
• Now if KE = Fd and if KE = ½mv2, then
Fd = ½mv2
Calculating Kinetic Energy
1. A ball with a mass of .15 kg is thrown at a
speed of 40.2 m/s. What is the ball’s KE?
2. If a car with a mass of 2000 kg is moving
at a speed of 25 m/s, what is it’s KE?
3. If a force of 1145N is applied to a railroad
boxcar and it rolls a distance of 85 meters,
what is it’s KE?
Calculating Kinetic Energy
1. KE = ½mv2 = (.5 x .15kg) x 40.2 m/s2
KE = .075kg x 1616.04 m/s = 121.2J
2. KE = ½mv2 = (.5 x 2000kg) x 25 m/s2
KE = 1000kg x 625 m/s = 625000J
3. KE = Fd = 1145N x 85m = 97325J
LAW OF CONSERVATION OF ENERGY
Nature never creates or destroys
energy; energy only gets converted
(changed) from one form to another.
Energy Transformation
PE
KE
PE
KE
Energy will continually
transfer between forms.
KE to PE
• PE and KE are
always equal to the
total amount of
energy
• PE and KE are in an
inverse relationship
– as one increases
the other decreases
Calculate the PE and the KE
Continual transfer between forms…
•
•
•
•
•
•
Mechanical Energy
Radiant (Electromagnetic) Energy
Electrical Energy
Chemical Energy
Nuclear Energy
Thermal Energy
TYPES OF ENERGY
Mechanical,
Electromagnetic,
Electrical, Nuclear,
Chemical and Thermal
What is Mechanical Energy?
o Energy due to a
object’s motion (kinetic)
or position (potential).
The bowling ball has
mechanical energy.
When the ball strikes
the pins, mechanical
energy is transferred to
the pins!
Examples of Mechanical Energy
What is Electromagnetic
Energy?
o Light energy
o Includes energy from
gamma rays, xrays,
ultraviolet rays, visible
light, infrared rays,
microwave and radio
bands
What is Electrical Energy?
o Energy caused by
the movement of
electrons
o Easily transported
through power lines
and circuits and
converted into other
forms of energy
What is Chemical Energy?
o Energy that is
available for release
from chemical
reactions.
The chemical bonds in
a matchstick store
energy that is
transformed into
thermal energy when
the match is struck.
Examples of Chemical Energy
What is Thermal Energy?
o Heat
energy
o The heat energy of an
object determines how
active its atoms are.
A hot object is one whose atoms
and molecules are excited and
show rapid movement.
A cooler object's molecules and
atoms will show less movement.
QUIZ TIME!
1.
What type of energy
cooks food in a
microwave oven?
2. What type of energy
is the spinning plate
inside of a microwave
oven?
QUIZ TIME!
Electrical energy is
transported to your house
through power lines.
3. When you plug an electric
fan to a power outlet,
electrical energy is
transformed into what type of
energy?
QUIZ TIME!
4. What energy
transformation occurs when
an electric lamp is turned on?
5. What types of energy are shown below?
6. What type of energy transformation is
shown below?
7. What types of energy are shown below?
8. What type of energy is shown below?
9. What type of energy conversion is shown
below?
bonusthequestion
• Draw a 50
flowpoint
map showing
flow of energy
transformations in a car from starting the
vehicle to driving. You should have 5
different types of energy transformations.
QUIZ TIME!
1. What type of energy
cooks food in a
microwave oven?
ELECTROMAGNETIC
ENERGY
2. What type of energy
is the spinning plate
inside of a microwave
oven?
MECHANICAL ENERGY
QUIZ TIME!
Electrical energy is
transported to your house
through power lines.
3. When you plug an electric
fan to a power outlet,
electrical energy is
transformed into what type of
energy?
MECHANICAL ENERGY
QUIZ TIME!
4. What energy
transformation occurs when
an electric lamp is turned on?
ELECTRICAL ENERGY

ELECTROMAGNETIC ENERGY
5. What types of energy are shown below?
Mechanical and Thermal Energy
(Don’t forget friction)
6. What type of energy transformation is
shown below?
Electromagnetic to Chemical Energy
7. What types of energy are shown below?
Chemical to Electrical, Mechanical to
Electromagnetic Energy
8. What type of energy is shown below?
Chemical Energy (yummy)
9. What type of energy conversion is shown
below?
Electrical to Thermal Energy
Energy TransferSound
(mechanical)
Thermal
Electrical
Mechanical
Chemical
Light
(Electromagnetic)