Post on 20-Jan-2016
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Ch 14 Machines
14.1 Machines help people do work
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Machines change the way force is applied
Machine—device that helps people do work. It does not change the amount of work done
• If a machine decreases the amount of force needed to do the work, the distance over which that force is applied increases
• A machine can change the direction of an applied force
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Input force—force exerted on a machine.
Output force—force exerted on an object by a machine
Mechanical advantage=# of times a machine multiplies the input force
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Work transfers energy
A machine increases the potential or kinetic energy of an object by doing work on it
*For a certain amount of work, if distance increases, force decreases
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Output work is always less than input work
Efficiency (%) = output work/input work X 100
No real machine is 100% efficient. Machines lose energy to friction, which is why we lubricate moving parts.
Another source of loss of efficiency is air resistance, which is the reason for str5eamlined designs for vehicles & cyclists’ helmets
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14.2 Six Simple Machines Have Many Uses
The lever & inclined plane are the 2 main types of simple machines
Other simple machines are based on these.
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Other Simple Machines
1. Lever—solid bar that rotates on a fixed point called the fulcrum
*There are 3 classes of levers based on the
relative locations of the input force, output
force, & fulcrum
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2. Wheel & axle—wheel is attached to a shaft & acts like a rotating collection of levers
*Input force can be applied to either part,
which transfers force to the other part
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3. Pulley—wheel with an axle & a grooved rim. A rope or cable moves in the groove. Pulleys can be fixed or movable. A combination of both is called a block & tackle
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4. Inclined plane—sloping surface that supports the weight of an object while the object moves from one level to another
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5. Wedge—has a thick & thin end; can be used to cut, split, pierce, or hold objects together
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6. Screw—incline plane wrapped around a cylinder or cone to form a spiral; can be used to hold things together or raise/lower objects
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The mechanical advantage of a machine can be calculated
If a machine were 100% efficient, its ideal mechanical advantage would be
output force divided by the input force
MA = F out
F in
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MA
Inclined plane
Divide the length of the incline by the height of the incline
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MA
Wheel & axle
Divide the radius where the input force is applied by the radius where the output force is applied
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MA
Lever
Calculate by dividing distance from input force to fulcrum by the distance from output force to fulcrum
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14. 3 Modern Technology Uses Compound Machines
• Compound machines are combinations of simple machines
• They often have many moving parts & must overcome more friction than simple machines
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Compound Machines (cont’d)
• Their MA equals the product of the MA of all the simple machines that make it up
• Example:
If a lever with a MA of 2 acts in series with a lever with MA of 3, the MA of the lever combo will be 2 x 3, or 6
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MA of a gear system comprising 2 wheel –and –axle systems…
Is found by dividing the # of teeth on the output wheel by the # of teeth on the input wheel.ExampleA wheel with 16 teeth turns another wheel with 24 teeth. The wheel with 16 teeth is the input wheel & the one with 24 teeth is the output wheel:
MA = 24 16 = 1.5
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Modern Technology Creates New Uses For Machines
• Sophisticated machines are often based on , or contain, several simple machines
• Machines built from individual atoms & molecules of material are the result of nanotechnology
*Most nanomachines are still in the experimental stage
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Robots are machines that work automatically or by remote control. They do jobs in places where it is difficult or dangerous for people to do work
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MA
Pulleys
MA is equal to the # of ropes that support the weight