Transcript Energy - EDL520

Energy
Physics
Energy
Energy comes to us from the sun.
Persons, places, and things have
energy, but we only observe the
effects of energy when something is
happening.
Energy can be transferred from one
form to another and from one place
to another.
Work
The concept of force x distance
is work.
When work is done two things
always occur:
– The application of a force
– The movement of something by
that force
Twice the load equals twice the
work because it requires twice
the force.
Twice the distance also equals
twice the work.
Types of Work
Work falls into two general categories.
The first category is work done against a
force.
You do work on something when you force
it to move against the indulgence of an
opposing force.
– Ex. Pulling a bowstring
The second is work done to change the
speed of an object.
– Ex. Speeding a car up.
Units of Work
The unit of measurement for work
combines a unit of force, N, with a
unit of distance, m.
The resulting unit of work is the
newton-meter, or joule, named after
James Joule.
One joule of work is done when a
force of 1 N is exerted over a distance
of 1 m.
There are also kilojoules, kJ (1000 J)
and megajoules, mJ (1,000,000 J).
Power
The definition of work does not include
time.
Power is the rate at which work is done.
– Power = Work Done/Time Interval
Twice the power means twice the work can
be done in the same amount of time.
The unit of power is called the watt, after
James Watt.
One watt of power is expended when one
joule of work is done in one second.
One horsepower is equal to 0.75 kW.
Mechanical Energy
Something that enables
an object to do work is
energy.
Energy is measured in
joules.
Energy occurs in many
forms
Two of the most common
forms of mechanical
energy are:
– Potential energy: energy do
to position
– Kinetic energy: movement
of something
Potential
Energy
An object may store energy by virtue of
its position.
The energy that is stored and held in
readiness is called potential energy (PE)
because in the stored state it has the
potential for doing work.
– Ex. Spring or Rubberband
Chemical energy is also potential energy.
Any substance that can do work through
chemical action possesses potential
energy.
Work and Potential Energy
Work is required to elevate objects against
the earth’s gravity.
The potential energy due to elevated
position is called gravitational potential
energy, GPE.
The amount of GPE an object possess is
equal to the work done against gravity in
lifting it or,
– GPE = weight x height = mgh
– Height is the distance above some chosen
reference level such as the ground.
Kinetic Energy
If an object is moving then it is capable of
doing work.
It has energy of motion, or kinetic energy,
KE.
The kinetic energy of an object depends
on the mass of the object as well as its
speed.
It is equal to half the mass multiplied by
the square of the speed or,
– Kinetic Energy = ½ mass x speed2
– KE = ½ mv2
Kinetic Energy
and Work
The kinetic energy of a moving object is
equal to the work required to bring it to
that speed from rest, or the work the
object can do while begin brought to rest.
The equation is net force x distance = KE,
– Or Fd = ½ mv2
Therefore, because speed is squared the
kinetic energy is quadrupled.
Conservation of Energy
Energy changes from one form to another.
It transforms without net loss or gain.
The study of various forms of energy
transformations led to the law of
conservation of energy.
The law states, “energy cannot be created
or destroyed. It can be transformed from
one form to another, but the total amount
of energy never changes.”
In any system, one quantity doesn’t
change: energy.
Machines
A machine is a device used to multiply
forces or simply to change the direction of
forces.
The concept that underlies every machine
is the conservation of energy.
If friction and heat is neglected than, work
input should equal work output, or
– (Force x distance)input=(Force x distance)output
Simple Machines
There are six types of simple
machines:
– The
– The
– The
– The
– The
– The
inclined plane
wedge
screw
lever
pulley
wheel and axle
Wedge
The wedge is a form of the inclined plane.
A wedge is an inclined plane that moves to
raise an object.
A wedge is usually a piece of wood or
metal that is thinner at one end.
The longer and thinner the wedge, the
more force it exerts.
Ex. Knife or Zipper
The Screw
The screw is an inclined plane wrapped
around a central bar or cylinder to form a
spiral.
A screw multiplies the force by acting
through a long distance.
The mechanical advantage of the screw
increases as the threads are closer
together.
Ex. Bolt and nut, and jar lids
The Inclined Plane
The inclined plane is a flat, slanted
surface.
It is a simple machine with no moving
parts that works by increasing the
distance over which the force is
exerted and thus multiplying the force.
The most common use is probably as
a ramp.
The Lever
A lever is a rigid bar that is free to
pivot, or move about, a fixed point.
The fixed point is called the fulcrum.
When a force is applied to part of the
bar by pushing or pulling, the lever
swings on the fulcrum and overcomes
the resistance force.
Types of Levers
There are three types of levers:
– First class lever: Resistance force
and effort force are on opposite
sides of the fulcrum
– Second class lever: the resistance
force and effort force are on the
same side with the effort force
further away from the fulcrum.
– Third class lever: the resistance
force and effort force are on the
same side with the resistance
force further away from the
fulcrum
Pulley
A pulley is a rope, belt or chain
wrapped around a grooved
wheel.
A pulley can function two
ways:
– Change direction of the force
– Change amount of the force
A fixed pulley (attached to a
stationary object) changes the
direction of the effort force.
A movable pulley is attached
to the object you are trying to
move and thus the effort force
is multiplied.
Fixed and movable pulleys can
be combined to accomplish
even more work.
Wheel and Axle
A wheel and axle is a simple
machine made up of two circular
objects of different sizes.
The wheel is the larger object
and it turns about the smaller
object called the axle.
Because the wheel is larger,
force is multiplied when it is
applied to the axle.
The mechanical advantage
depends on the radius of the
wheel and axle systems.
Ex. Screwdriver, Ferris Wheel
Mechanical Advantage
The ratio of output force to input
force for a machine is called the
mechanical advantage.
Neglecting friction, the mechanical
advantage can also be determined by
the ratio of input distance to output
distance.
Efficiency
An ideal machine would be 100%
efficient.
However, in any machine, some
energy is transformed into atomic or
molecular kinetic energy aka heat.
Efficiency will always be less than
one.
Efficiency is expressed as,
– Useful work output/total work output
Rube Goldberg
Rube Goldberg was a Jewish American
cartoonist who became famous for his
Rube Goldberg machines.
He drew cartoons for the many
newspapers including the New York
Evening Journal and won the Pulitzer Prize
in 1948.
Rube Goldberg machines were developed
to poke fun at the way useful devices are
supposed to make life easier.
Rube Goldberg Machines
A Rube Goldberg machine is an
exceedingly complex machine that
performs a simple task in a
convoluted way.
In 1987, Purdue University began
the National Rube Goldberg machine
contest in which college teams build
machines around a simple theme.
An Example
The Task
Your task is to build a Rube Goldberg
machine to complete the simple task of
making a cup of coffee.
Your machine must be small enough to fit
through the classroom door and complete
the task in less than nine minutes.
You must use at least 15 steps to
complete the task and use each of the six
simple machines.
You will also have to submit a drawing and
written description of your machine
outlining each energy transfer.