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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