 # Chapter 12 – Work and Machines

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Chapter 12 – Work and Machines
Work is when a force is used to make something move. Work = Force (Distance) Work units English units will be foot.pounds Metric units will be newton.meters (which is called a joule)

Example problem: Use the four step method to determine how much work is done if a 50 newton force is used to drag a rock 30 meters. The force used in this equation is the force in the direction of motion. 40 newtons Force 30 newtons Force 30 newtons Force used to move box

Machines A machine is a device with which you can do work in a way that is easier or more effective A machine can be simple or complex A machine makes work easier by: Changing the amount of force you exert or Changing the distance over which you exert the force The direction in which you exert your force

Input force – force you exert on the machine
Output force – force exerted by the machine Output Force Mechanical advantage = Input Force Multiplying Force (mechanical advantage greater than 1) If the input force is less than the output force, the input force must be exerted over a greater distance. Examples: a car jack, pulling nails with a hammer

Multiplying distance (mechanical advantage less than 1)
If the input force is greater than the output force, the output end of the machine will move a greater distance. Examples: a fan, a hockey stick, high gear on a bicycle or car Changing Direction of Force Example: a pulley, a lever

Efficiency Efficiency is the total work or energy input into a machine compared to the useful work or energy output of the machine Output Work Efficiency = x 100 Input Work

If there was no energy loss through friction or other causes, the efficiency of a machine would be 100% No machine is 100% efficient, although some simple machines are very close to 100% efficient

For simple machines Input work ≈ Output work Lift a distance of 2 ft.
Push down a distance of 5 ft. 20 lbs 50 lbs Input work = F (D) Input work = 20 lb (5 ft.) Input work = 100 ft-lbs Output work = F (D) Output work = 50 lb (2 ft.) Output work = 100 ft-lbs 100 ft-lbs ≈ 100 ft-lbs

Use the efficiency equation to determine the approximate efficiency of the lever system.
output work Efficiency = x 100 = input work 100 ft-lbs x efficiency = 100%

Example efficiency problems: Use the four step method to solve these problems
Determine the efficiency of a machine that requires a work input of 224 foot pounds for a work output of 200 foot pounds. A hydraulic jack requires thirty ½ foot down ward strokes of 20 pounds each to lift an 1150 pound object a distance of .25 feet. Calculate the efficiency of the jack.

Ideal Mechanical Advantage – the mechanical advantage of a machine if there was no friction. This can be calculated. Actual Mechanical Advantage – the real mechanical advantage of a machine. This can be different for each machine and must be measured.

Part 2 – Simple Machines The 6 types of simple machines are:
The inclined plane The wedge The screw The lever The wheel and axle The pulley

The Inclined Plane Mechanical advantage (MA)= output force/input force
Ideal MA = length of the incline/height of incline If there was no friction the actual MA would equal ideal MA. In real life it is always less. Incline length Incline height

Wedge A wedge is like a moving inclined plane to split or cut things apart.

Screw A screw is also related to the inclined plane.
The threads of a screw are like a spiral inclined plane. The closer the threads, the greater the MA of the screw.

Levers (3 types) First Class Lever (a pry or teeter-totter)
Second class Lever (wheel barrel) Third class Lever Output force Input force Input force Output force Input force Output force Distance from fulcrum to input force Ideal MA = Distance from fulcrum to output force

Wheel and Axle The wheel and axle are fastened together so they rotate together (door knob, screw driver, steering wheel of a car) radius of wheel mechanical advantage = radius of axle

Pulley A single fixed pulley has a mechanical advantage of one.