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MECHANISMS I: SIMPLE MACHINES INDEX 1) PRELIMINARIES ....................................................................................................................... 1 a) Mechanical advantage .............................................................................................................. 2 b) Conservation of energy ............................................................................................................ 2 c) The three (six?) Simple machines ............................................................................................ 2 2) LEVER ......................................................................................................................................... 3 a) First-Class Lever. ..................................................................................................................... 3 b) Second-Class Levers. ............................................................................................................... 3 c) Third-Class Lever. .................................................................................................................... 3 3) WHEEL AND AXLE .................................................................................................................. 4 4) PULLEY ...................................................................................................................................... 4 a) Fixed pulley .............................................................................................................................. 4 b) Moveable pulley ....................................................................................................................... 4 c) Block and tackle ....................................................................................................................... 5 How does a Block and tackle work? ............................................................................................ 5 5) INCLINED PLANE ..................................................................................................................... 6 6) WEDGE ....................................................................................................................................... 6 7) SCREW ........................................................................................................................................ 7 1) PRELIMINARIES All machines have in common the following things: they involve a kind of motion they involve a kind of force they make a job easier to do they need some kind on input to make them work they produce some kind of output The four basic kinds of motion are: Rotary: going round and round. This is the most common kind of movement Oscillating, swinging backwards and forwards Linear, in a straight line Reciprocating, backwards and forwards in a straight line A machine is a device that helps make work easier to perform. (Remember, Work = Force X Distance). A machine makes work easier to perform by accomplishing one or more of the following functions: transferring a force from one place to another, changing the direction of a force, increasing the magnitude of a force, or increasing the distance or speed of a force. IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 1 a) Mechanical advantage When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced. If a machine increases an input force of 10 newton to an output force of 100 newton, the machine has a mechanical advantage (MA) of 10. This is shown below: MA = Output Force/Input force = 100/10 = 10 b) Conservation of energy No machine can increase both the magnitude and the distance of a force at the same time. When a machine produces an increase in force, there is always a proportional decrease in the distance moved. Conversely, when a machine produces an increase in distance, there will be a proportional decrease in force. Another way to state this concept is that no machine can produce more work than the amount of work that is put into the machine. In fact, This concept can also be put into the form of an equation. (Remember that work is equal to force times distance.) F1 X D1 = F2 X D2 Where: F1 = Input Force, F2 = Output Force D1 = Input Distance D2 = Output Distance c) The three (six?) Simple machines You are probably familiar with many different machines. Some of these machines appear highly complex. However, all machines, no matter how complex, are made up of one or more of the six simple machines. The six simple machines are: Lever Wheel and Axle Pulley Inclined Plane Wedge Screw Individually, each of these machines is a simple machine. When two or more simple machines are combined in such a way that they work as a single mechanism, the device is classified as a complex machine. IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 2 2) LEVER A lever is a rigid bar that rotates around a fixed point called the fulcrum. The bar may be either straight or curved. In use, a lever has both an applied force and a resistance force. There are three different classes of levers. The class of a lever is determined by the location of the applied and resistance forces relative to the fulcrum. Each of the three classes of levers will be discussed next. a) First-Class Lever. The fulcrum is located at some point between the effort and resistance forces. Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws. When the fulcrum is nearer the weight, you have to apply a smaller effort, and reversely when the fulcrum is nearer where you apply the force, then this force must be a bigger effort to balance the weight b) Second-Class Levers. With a second-class lever, the resistance is located between the fulcrum and the effort force. Common examples of second-class levers include nut crackers, wheel barrows, and bottle openers. This levers are said to be always “advantageous” because you always have to apply a smaller effort to balance the weight c) Third-Class Lever. With a third-class lever, the effort force is applied between the fulcrum and the resistance force. Examples of third-class levers include tweezers, ice tongs, and shovels. Common examples of third-class levers include a fishing pole. This levers are said to be always disadvantageous because you always have to apply a bigger effort to balance the weight IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 3 3) WHEEL AND AXLE The wheel and axle is a simple machine consisting of a large wheel rigidly secured to a smaller wheel or shaft, called an axle. When either the wheel or axle turns, the other part also turns. So that, one full revolution of either part causes one full revolution of the other part. If the wheel turns and the axle remains stationary, it is not a wheel and axle machine. The mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle. In the wheel and axle illustrated below, the radius of the wheel is five times larger than the radius of the axle. Therefore, the mechanical advantage is 5:1 . The wheel and axle can be used to increase speed. This is done by applying the input force to the axle rather than a wheel. 4) PULLEY A pulley consists of a grooved wheel that turns freely in a frame called a block. A pulley can be used to: simply change the direction of a force or to gain a mechanical advantage, depending on how the pulley is arranged. a) Fixed pulley A pulley is said to be a fixed pulley if it does not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage. A fixed pulley is illustrated below. b) Moveable pulley A moveable pulley rises and falls with the load that is being moved. A single moveable pulley creates a mechanical advantage; however, it does not change the direction of a force. IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 4 c) Block and tackle In many applications, both fixed and moveable pulleys are used in combination to form a device known as a block and tackle. A block and tackle is capable of both changing the direction of a force and creating a mechanical advantage. How does a Block and tackle work? A block and tackle can contain as many pulleys as you like, although at some point the amount of friction in the pulley shafts begins to become a significant source of resistance. Imagine that you have the arrangement of a 100 pound (45.4 kilogram) weight suspended from a rope, as shown on the right. Here you have to apply an upward force of 100 pounds to the rope. If the rope is 100 feet (30.5 meters) long and you want to lift the weight up 100 feet, you have to pull in 100 feet of rope to do it. This is simple and obvious. Now imagine that you add a pulley to the mix, as shown on the right. Does this change anything? Not really. The only thing that changes is the direction of the force you have to apply to lift the weight. You still have to apply 100 pounds of force but maybe more comfortably. The following figure shows the arrangement after adding a second pulley. That is a block and tackle system. The force has been cut in half but the distance the rope must be pulled has doubled. The following diagram adds a third and fourth pulley to the arrangement. This makes even easier to lift the weight (you have to apply just 25 pounds), but having to lift it more distance (if you want to lift it 100 feet, you have to pull 400 feet. Detail to see how the rope surrounds the third pulley IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 5 5) INCLINED PLANE An inclined plane is an even sloping surface. The inclined plane may slope at any angle between the horizontal and the vertical. The inclined plane makes it easier to move a weight from a lower to higher elevation. An inclined plane is illustrated below: The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane. While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move. 6) WEDGE The wedge is a modification of the inclined plane. Wedges are used as either separating or holding devices. Like and axe to cut off trees or a wedge to hold a door open. IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 6 7) SCREW The screw is also a modified version of the inclined plane. While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane). It is like having to walk up a longer distance with a smaller effort (like the inclined plane) but doing it in s small space. Examples of application of this simple machine are the screws, spiral staircases or the Archimede’s screw for transporting things. IES PROFESOR MÁXIMO TRUEBA - Curso 2009-10 ........... 1º ESO - Dpto. Tecnología. Sección bilingüe Worktext. Mechanisms I: simple machines......................................................................................... Página 7