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Chapter 12 – Work and Machines

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1 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)

2 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

3 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

4 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

5 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

6 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

7 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

8 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

9 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%

10 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.

11 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.

12 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


14 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



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



20 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.


22 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




26 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


28 Pulley A single fixed pulley has a mechanical advantage of one.
A single movable pulley has a mechanical advantage of two We can add more movable pulleys to add more mechanical advantage.


30 Compound Machine A machine that uses two or more simple machines Identify the simple machines in each of the following.

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