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Mechanical Systems
Unit Review
Early Machines
• machines help us do work and use energy more
efficiently
• early machines were simple devices to help us
do work such as: lift things, move things, crush
things, or split things
Industrial Revolution
• now we had power such as steam, coal,
oil…
• and we could improve the ability of
machines to do work for us
Simple Machines
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•
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levers
winches
incline planes
wedges
screws
pulleys
• complex machines are several simple
machines working together
Levers
• simple machine that changes the amount
of force needed to move an object
• made up of a bar that is free to rotate
around a fixed point called a fulcrum
• force exerted on the lever to make it move
is the effort force
• the mass of the object being moved is
called the load
• distance between fulcrum and load is
called the load arm
3 Classes of Levers
E
L
L
L
F
F
F
E
E = Effort Force
L = Load
F = Fulcrum
E
Winches
• consists of a small cylinder that has a
crank or handle to produce effort force.
• axle of the winch acts like the fulcrum
• by exerting a force on the handle to turn
the wheel the cable retracts the load
Incline Plane
• a ramp or slope that reduces the force
needed to lift something
• it does this by increasing the distance over
which the work is done
Screw
• its an incline plane that is wound around a
central post
• helps to increase the force used
– by rotating the screw, the incline plane pushes it
forward (or into something), & the screw slowly
travels a straight distance
• examples
– screw
– spiral staircase
– jar lid
Wedge
• similar in shape to an incline plane
• works only in one direction
• when pushed into an object it gradually
splits it apart
– e.g. axe head splitting wood or a knife cutting
something
Pulleys
• consists of wire, rope or chain moving on a
grooved wheel
• designed to help lift large loads
• fixed pulley is attached to something that does
not move (e.g. ceiling)
• moveable pulley is attached to something else,
but the pulley can move
• block and tackle uses a
combination of fixed and
moveable pulleys to
make a “supercharged”
system that can move
very heavy loads
• when multiple pulleys are
used in a system its
called a compound
pulley
Wheel and Axle
• combination of two wheels of different
diameters that turn together
– one is called the wheel… the other the axle
– the longer the motion on the wheel… the
greater the force on the axle… the more work
gets done
Complex Machines
• several simple machines working together
• a system is a group of parts that work
together to perform a function
• subsystems are usually simple machines
• linkages for the network or link between
the subsystems so the system works
A Bicycle is a Complex Machine
Brakes in a Car are a Complex
Machine
Gears
• gears are an essential component of most
mechanical systems BECAUSE they
transfer energy
• consist of a pair of wheels with teeth that
interlink
• can be used to increase or decrease
speed of the mechanical system
• can be used to change the direction of
motion of the mechanical system
A smaller gear (Y) is called a pinion.
The gear that supplies the energy is called the driving gear (X).
The gear to which the force is directed is called the driven gear (Y).
A large gear (X) driving a smaller gear (Y)
decreases torque and increases speed in the driven gear.
Gears such as these are called multiplying gears.
A small gear (Y) driving a larger gear (X) increases torque
and reduces speed in the driven gear.
Gears like these are called reducing gears.
When the driving gear has fewer teeth than the driven gear, the driven gear then
rotates more slowly than the driving gear. A car or bicycle in low gear uses reducing
gears.
When the driving and the driven gears are the same size they are known as
parallel gears.
Work
• work is energy in action
– if there is no movement of object there is no work
• Work = Force x Distance
– how much force was applied, and for how far
• work is measured in Joules (J)
• work input is the work you apply on the
machine
• work output is the work the machine does on
the load
Mechanical Advantage
• is a comparison of the force produced by a
machine to the force applied to the machine (by
you)
• Mechanical Advantage (MA) = Load force (FL) or
Effort force (FE)
Output Force
Input Force
• or for levers, MA = effort arm
load arm
• load force and effort force are both measured in
Newtons (N), therefore MA has no units of
measure
• MA can be <1, 1 or >1
Mechanical Advantage - Pulleys
• to calculate the MA of a pulley, count the
number of ropes/cables supporting the
load
MA <1
• when MA is >1 the machine is multiplying
the input force to create a larger output
force
• when MA is <1 it is useful for tasks that
don’t require a large output force
– e.g. a bicycle is a machine with a MA <1
• the output force causes the bicycle to move faster
than the rider could walk, so it is a very useful
machine
Friction
• the difference between the calculated value and
the real (actual) value of mechanical advantage
is friction
• friction is essentially a force that opposes motion
• it is caused by the roughness of materials
• force is always needed to overcome friction
– so the MA of the device will be less because of this
added force that must be overcome
• friction in a system causes heat
Speed Ratio
• speed measures the distance an object
travels in a given amount of time
• the measure of how a machine affects
speed is called the speed ratio (SR)
• SR = input distance OR d input (m)
output distance
d output (m)
Efficiency
• efficiency is a measure of how well a machine or a
device uses energy
– the more energy that is lost, the less efficient a machine is
– friction is a main factor in loss of energy
– efficiency is represent in %.
• Efficiency (%) = Mechanical Advantage X 100
Speed Ratio
• lubricants can be used to reduce friction and improve
efficiency
• NO MACHINE CAN BE 100% EFFICIENT !!!!
Hydraulic Systems
• liquids are placed under pressure in a confined
space
– because they are relatively incompressible… the can
transmit that pressure to do work… like lifting very
heavy objects
• pressure in liquids is measured in Pascals (Pa):
• P = F/A
– F is force (N)
– A is area (m2)
Pressure
Lift
Pneumatic Systems
• air or gas is placed under pressure in a
confined space…
• operates essentially like a hydraulic
system
• e.g. sand blaster, air gun, nail gun,
jackhammer, hovercraft