Transcript Simple Machines

Machines
Mechanical Advantage, Efficiency,
and Types of Simple Machines.
http://video.google.com/videoplay?docid=-4187430023476942057
Machines
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A machine is any device that helps you do work.
Machines do not reduce the amount of workthey multiply the effort that is applied or they
multiply the distance over which the force is
applied.
Machines can be classified as simple,
compound, or complex based on the number of
parts.
Rube Goldberg Machine
Mechanical Advantage
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The force you exert on a machine is called the
effort force (Fe).
The force exerted by the machine is called the
resistance force (Fr).
Mechanical advantage (MA) is the ratio of the
resistance force to the effort force, Fr/ Fe.
Ideal mechanical advantage (IMA) is the ratio of
effort distance (de) to the resistance distance
(dr), de/dr.
Efficiency
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Efficiency is the ratio of output work to input
work.
Due to friction between moving parts, no
machine has an efficiency of 100%.
Efficiency = Wout / Win x 100%
Efficiency = MA / IMA x 100%
Review
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If you increase the efficiency of a machine,
does the
a) MA increase, decrease, or remain the
same?
b) IMA increase, decrease, or remain the
same?
Types of Simple Machines
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Inclined Plane
Wedge
Screw
Lever
Pulley
Wheel and Axle
Inclined Plane
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An inclined plane is a flat
surface set at an angle against
a horizontal surface.
Inclined planes allow you to
apply a smaller force by
increasing the distance over
which the force is applied.
As the angle of the slant
decreases, the mechanical
advantage increases.
Some examples of an
inclined plane are the
playground slide, steps, a ski
jump, and a wheelchair
ramp.
Wedge
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A wedge is two inclined
Examples of a wedge
planes joined back to
are an ax, the point of a
back.
nail, a doorstop, and a
When you use an
knife
inclined plane to do work,
the inclined plane stays
 One end of the wedge
still and the object being
tapers to a thin edge and
acted upon is moving.
the other end is wide. The
When using the wedge,
longer and thinner a wedge
the object being acted
is, the less effort is needed
upon stays still and the
wedge is moving
to overcome the resistance
force.
Screw
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A screw is an inclined plane
wrapped around a cylinder.
A screw makes work easier by
allowing the work to be done over
a longer distance, just as an
inclined plane does.
The distance between the
threads is called the pitch.
The closer together the threads of
a screw are, or the smaller the
pitch, the longer the distance over
which the effort is exerted, and
the more the force is multiplied.
Examples of a screw
include the lid of your
jar of peanut butter,
the jack you lift the car
with when changing a
tire, and airplane
propellers.
Levers
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A lever is a rigid bar that
rests on a fulcrum (a
fixed point.)
The farther the force is
from the fulcrum, the
easier it is to work the
lever.
Levers may change the
direction as well as the
amount of force.
There are three classes
(kinds) of levers
First Class Lever
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A first class lever has its
fulcrum located
somewhere between the
effort and the resistance
(load).
The direction of force is
changed with this type of
lever. Applying effort
downward moves the load
up and applying effort
upward moves the load
down.
Examples of first class
levers that you may be
familiar with are the
playground seesaw, a
crowbar, scissors, and
pliers
Second Class Lever
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With a second class lever,
the fulcrum is at one end,
the effort is at the other
end and the resistance is
in the middle.
With this kind of lever, the
direction of effort is not
changed. Pushing up on
the lever arm pushes up
on the load.
Common examples of
second class levers are
a screwdriver, a
catapult, a nutcracker,
a wheelbarrow, and a
stapler
Third Class Lever
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The fulcrum is at one end
and the effort is applied
between the fulcrum and
the resistance.
With this kind of lever, the
direction of effort is not
changed. The load moves
in the same direction as
the effort.
Examples of third class levers
include baseball bats, hockey
sticks, tennis rackets, golf clubs,
brooms, and fishing poles.
Pulley
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A pulley is a rope, belt,
or chain wrapped
around a grooved
wheel.
The pulley is a
variation of the lever.
Pulleys can be fixed or
moveable.
Pulley
Examples of a
pulley can be found
at the top of a
flagpole. You raise
and lower your
window blinds with a
pulley.
Fixed Pulley
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A pulley that is attached
to a structure is called a
fixed pulley.
The wheel of a fixed
pulley turns, but the
pulley itself does not
move.
A fixed pulley does not
multiply the effort force.
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The distance you apply
the effort is the same as
the distance the load
moves.
A fixed pulley changes
the direction of effort.
When you pull down on
the rope, the load moves
up.
Pulling down is easier
than pulling up because
you use your body weight
when pulling down.
Moveable Pulley
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A moveable pulley is attached to the object you are moving.
One end of the rope is attached to a fixed structure overhead.
The other end of the rope goes down through the pulley
attached to the load and then back up to the top. Pulling on
the other end of the rope causes the load to move up.
The moveable pulley offers a mechanical advantage even
though it does not change the direction of effort.
The load is supported by rope on both sides of the pulley,
which means that half as much effort is needed to lift the
load. You must exert effort twice as far as the load moves.
The force needed to move an object is less, but the distance
through which the force must move is longer.
Wheel and Axle
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A wheel and axle is a simple
machine made up of two circular
objects of different size.
The axle (a small wheel) is
attached to the center of a larger
wheel. All wheels need an axle.
If the radius of the wheel is two
times larger than the radius of the
axle, every time you turn the wheel
once, the force will be multiplied by
two.
A wheel and axle can produce a
gain in either effort or distance,
depending on how it is used.
Examples of a wheel and
axle can be found in
doorknobs, roller skates,
and the handles of a faucet.
Review
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Identify the type of simple machine that
each of the following tools are examples of.
a) screwdriver
b) pliers
c) chisel
d) staple remover
e) wrench
Compound Machines
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A compound machine
consists of two or more
simple machines linked
so that the resistance
force of one machine
becomes the effort force
of the second.
Examples include a
pencil sharpener, a can
opener, and a bicycle.
Example 1:
A pulley is used to raise a heavy crate. If the
crate has a weight of 1784 N and you must
supply a force of 223 N to lift the crate, what
is the mechanical advantage of the pulley?
Example 2:
A lever and a fulcrum are used to raise a
heavy rock, which has a weight of 445 N. If
the lever arm has a mechanical advantage of
9.5, what must be the input force on the lever
in order to lift the rock?
Example 3:
It has been proposed that the stones of the
Pyramids in Egypt were raised by using
ramps. Suppose one of these ramps has a
mechanical advantage of 3.86. If an effort
force of 6350 N was provided by laborers,
what would the resistance force of the stone
have been?
Example 4:
A mover uses a ramp to load a heavy crate
onto a truck. The crate must be lifted a
vertical distance of 1.4 m from the street to
the bed of the truck. If the ramp is 4.6 m long
and friction can be ignored, what is the
mechanical advantage of the ramp?