Transcript p.s.-chap-12-work-energy-and-simple-machines

Work,Machines,
and Energy
Work
and Power
Simple Machines
Mechanical Advantage
Work
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Two conditions must be met in order for
work to be done on an object:
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The object moves.
A force must act on the object in the direction
the object moves.
Work= force * distance
Problem
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Example: How much work is completed
when a digger uses 26,000 newtons to
dig 6 meters down in the dirt?
Work= force* distance
26,000 newtons * 6 meters=
156000 joules
Power=work/time
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Power is the rate at which work is done.
Example: How much power was done if
there was1253.4 joules of work completed
in 3600 seconds?
Power= work/time
Power = 1253.4joules =
3600 seconds
0.34816 watts
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Example:
If a receiver catches the ball at his
20-yard line. He then runs towards the
goal. At the opponents 10 yard line he
makes a U-turn and runs back to his 20yard line, where he started.
Did he do any work?
No, he did not move.
His distance was zero.
10-yard line
50 yard line
20-yard line
Machines
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A machine is a device that makes work
easier by changing the direction or the
amount of force needed to do the work.
The force applied to the machine is
referred to as the effort force.
The force opposing the effort is called
the resistance force or “load”.
Mechanical energy
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Is the energy of movement or position of
an object.
It is also called Kinetic energy.
Wind and moving water have Mechanical
energy.
Other examples: windmills, pencil
sharpener, Chain-fall,Tire-jack
Simple Machines
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Simple machines are devices that do one
movement.
There are six simple machines that make
up all of the compound machines.
Compound machines have two or more
simple machines, such as a computer.
Simple Machines
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The six simple machines are:
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Inclined plane
Levers
Pulleys
Wheel and Axle
Screw
Wedge
Inclined plane
First class levers
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First class levers:multiply the effort force
and change the direction.
Other examples of 1st class
levers
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Other examples:
shears,
seesaw,scissors, pliers,
shovel
Second class levers
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On a 2nd class levers the distance is
increased.
Force is multiplied without changing
the direction.
Other 2nd class levers
 Doors,
bottle openers,
nutcrackers,
Third class levers
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In a 3rd class lever, the effort force is greater
than the resistance force.
The Mechanical advantage is less than one.
A 3rd class lever is used to increase the
distance moved, not to reduce the force.
Other 3rd class lever examples
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Rake, broom, fishing pole
Pulleys
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A pulley is a rope wrapped around a grooved
wheel.
There are fixed and movable pulley systems.
The fixed pulley always has a Mech.
Advantage of one. (It only changes the
direction)
A movable pulley has a Mech Advantage of
greater than one.
Fixed Pulley
M.A.=1
Movable Pulley
M.A. of 2
2 ropes supporting
the load
Movable Pulley
M.A. = 3
3 ropes
supporting the load
Wheel and axle
The M.A. is always greater than one. Other examples
are a door knob, a Ferris wheel,a wheel chair.
Screw
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A screw is an inclined plane wrapped
around a shaft. Water faucets, lids, etc.
Wedge
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Is an inclined plane used to push objects
up or apart.
Examples; knives, razors,chisels, axes,
splitting logs (wedge), saws
Mechanical Advantage
Most machines multiply the force of
your efforts. The number of times a
machine multiplies an effort force is
called Mechanical Advantage (M.A.).
 A machine with a M.A. of 2 doubles
your effort force.
 Machines can also change the
direction of the force.
 If a machine has a M.A. of less than 1,
increases the distance or the speed of
motion. (remember that it will still
decrease the resistance force.)
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Mechanical Advantage
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Tells how much a machine multiplies force
or increases distance
Mechanical Advantage = output force/input
force
OR MA= Input Distance/Output
Distance
Mechanical efficiency
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Mechanical efficiency = work output * 100%
work input
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Work output: amount of work done by the
machine.
Work input: amount of work you apply to the
machine.
Mechanical efficiency will always be less than
100% since the work output is always less then
the work input.
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Mechanical efficiency
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Mechanical efficiency is always less than
100% due to the following factors:
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Heat loss, body, car engine, all motors, etc.
Friction
Sound & vibration
Energy changes
With all motion energy is required.
 When an object moves it has
Kinetic energy (motion).
 When an object is standing still it
has Potential energy.
 A burning match has Kinetic
energy. The amount equals the
Potential energy that was stored in
the match before it was lighted.
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A
battery before it is used has
Potential energy.
 Food setting on a shelf has Potential
energy.
 Kinetic energy= Mass* velocity2
2
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Kinetic energy increases with an
increase in mass and/or velocity.
Law of Conservation of energy
Energy cannot be created or
destroyed.
 Energy can change forms.
 Such as mechanical energy of a hand
changing to electrical when it turns a
hand generator.
 Or mechanical energy changing to
sound energy in a speaker.
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Energy
Energy - The ability to cause
changes.
 Forms of energy:
 Radiant
 Electrical
 Chemical
 Thermal
 Nuclear
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Potential energy, energy of
position or condition
 A special type of potential energy
is called gravitational potential
energy.
 G.P.E. - dependent upon the
height above the earth’s surface
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Conservation of Energy
 Energy
can not be created or
destroyed.
 Mechanical energy
 Total amount of kinetic and
potential energy in a system.
http://science.howstuffworks.com/
roller-coaster2.htm
Work
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Define:
The transfer of energy through motion.
A force must be exerted through a
distance in order for work to be done.
Work = force x distance or
W=Fxd
Work
Work is measured in joules.
 1 joule equals a
newton – meter (N• m)
 Joule - amount of work done when
a force of one newton acts through
a distance of one meter.
 When 1 joule of work is done on an
object, 1 joule of energy has been
transferred to the object.
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Work
 When
describing work it is
important to identify on what the
work is being done.
 For work to be done, something
must move and the movement is
in the direction of the applied
force.