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Machines, Simple

Machines, simple

A simple machine is a device for doing work that has only one part. Simple machines redirect or change the size of forces, allowing people to do work with less muscle effort and greater speed, thus making their work easier. There are six kinds of simple machines: the lever, the pulley, the wheel and axle, the inclined plane, the wedge, and the screw.

Everyday work

We all do work in our daily lives and we all use simple machines every day. Work as defined by science is force acting upon an object in order to move it across a distance. So scientifically, whenever we push, pull, or cause something to move by using a force, we are performing work. A machine is basically a tool used to make this work easier, and a simple machine is among the simplest tools we can use. Therefore, from a scientific standpoint, we are doing work when we open a can of paint with a screwdriver, use a spade to pull out weeds, slide boxes down a ramp, or go up and down on a see-saw. In each of these examples we are using a simple machine that allows us to achieve our goal with less muscle effort or in a shorter amount of time.

Earliest simple machines

This idea of doing something in a better or easier way or of using less of our own muscle power has always been a goal of humans. Probably from the beginning of human history, anyone who ever had a job to do would eventually look for a way to do it better, quicker, and easier. Most people try to make a physical job easier rather than harder to do. In fact, one of our human predecessors is called Homo habilis, which means "handy man" or "capable man." This early version of our human ancestors was given that name because, although not quite fully human, it had a large enough brain to understand the idea of a tool, as well as hands with fingers and thumbs that were capable of making and using a tool. Therefore, the first simple machine was probably a strong stick (the lever) that our ancestor used to move a heavy object, or perhaps it was a sharp rock (the wedge) used to scrape an animal skin, or something else equally simple but effective. Other early examples might be a rolling log, which is a primitive form of the wheel and axle, and a sloping hill, which is a natural inclined plane. There is evidence throughout all early civilizations that humans used simple machines to satisfy their needs and to modify their environment.

Words to Know

Compound machine: A machine consisting of two or more simple machines.

Effort force: The force applied to a machine.

Fulcrum: The point or support on which a lever turns.

Resistance force: The force exerted by a machine.

Work: Transfer of energy by a force acting to move matter.

The beauty of simple machines is seen in the way they are used as extensions of our own muscles, as well as in how they can redirect or magnify the strength and force of an individual. They do this by increasing the efficiency of our work, as well as by what is called a mechanical advantage. A mechanical advantage occurs when a simple machine takes a small "input" force (our own muscle power) and increases the magnitude of the "output" force. A good example of this is when a person uses a small input force on a jack handle and produces an output force large enough to easily lift one end of an automobile. The efficiency and advantage produced by such a simple device can be amazing, and it was with such simple machines that the rock statues of Easter Island, the stone pillars of Stonehenge, and the Great Pyramids of Egypt were constructed. Some of the known accomplishments of these early users of simple machines are truly amazing. For example, we have evidence that the builders of the pyramids moved limestone blocks weighing between 2 and 70 tons (1.8 and 63.5 metric tons) hundreds of miles, and that they built ramps over 1 mile (1.6 kilometers) long.

Trade-offs of simple machines

One of the keys to understanding how a simple machine makes things easier is to realize that the amount of work a machine can do is equal to the force used, multiplied by the distance that the machine moves or lifts the object. In other words, we can multiply the force we are able to exert if we increase the distance. For example, the longer the inclined planewhich is basically a rampthe smaller the force needed to move an object. Picture having to lift a heavy box straight up off the ground and place it on a high self. If the box is too heavy for us to pick up, we can build a ramp (an inclined plane) and push it up. Common sense tells us that the steeper (or shorter) the ramp, the harder it is to push the object to the top. Yet the longer (and less steep) it is, the easier it is to move the box, little by little. Therefore, if we are not in a hurry (like the pyramid builders), we can take our time and push it slowly up the long ramp to the top of the shelf.

Understanding this allows us also to understand that simple machines involve what is called a "trade-off." The trade-off, or the something that is given up in order to get something else, is the increase in distance. So although we have to use less force to move a heavy object up a ramp, we have increased the distance we have to move it (because a ramp is not the shortest distance between two points). Most primitive people were happy to make this trade-off since it often meant being able to move something that they otherwise could not have moved.

Today, most machines are complicated and use several different elements like ball bearings or gears to do their work. However, when we look at them closely and understand their parts, we usually see that despite their complexity they are basically just two or more simple machines working together. These are called compound machines. Although some people say that there are less than six simple machines (since a wedge can be considered an inclined plane that is moving, or a pulley is a lever that rotates around a fixed point), most authorities agree that there are in fact six types of simple machines.

Lever

A lever is a stiff bar or rod that rests on a support called a fulcrum (pronounced FULL-krum) and which lifts or moves something. This may be one of the earliest simple machines, because any large, strong stick would have worked as a lever. Pick up a stick, wedge it under one edge of a rock, and push down and you have used a lever. Downward motion on one end results in upward motion on the other. Anything that pries something loose is also a lever, such as a crow bar or the claw end of a hammer. There are three types or classes of levers. A first-class lever has the fulcrum or pivot point located near the middle of the tool and what it is moving (called the resistance force). A pair of scissors and a seesaw are good examples. A second-class lever has the resistance force located between the fulcrum and the end of the lever where the effort force is being made. Typical examples of this are a wheelbarrow, nutcracker, and a bottle opener. A third-class lever has the effort force being applied between the fulcrum and the resistance force. Tweezers, ice tongs, and shovels are good examples. When you use a shovel, you hold one end steady to act as a fulcrum, and you use your other hand to pull up on a load of dirt. The second hand is the effort force, and the dirt being picked up is

the resistance force. The effort applied by your second hand lies between the resistance force (dirt) and the fulcrum (your first hand).

Pulley

A pulley consists of a grooved wheel that turns freely in a frame called a block through which a rope runs. In some ways, it is a variation of a wheel and axle, but instead of rotating an axle, the wheel rotates a rope or cord. In its simplest form, a pulley's grooved wheel is attached to some immovable object, like a ceiling or a beam. When a person pulls down on one end of the rope, an object at the opposite end is raised. A simple pulley gains nothing in force, speed, or distance. Instead, it only changes the direction of the force, as with a Venetian blind (up or down). Pulley systems can be movable and very complex, using two or more connected pulleys. This permits a heavy load to be lifted with less force, although over a longer distance.

Wheel and axle

The wheel and axle is actually a variation of the lever (since the center of the axle acts as the fulcrum). It may have been used as early as 3000 b.c., and like the lever, it is a very important simple machine. However, unlike the lever that can be rotated to pry an object loose or push a load along, a wheel and axle can move a load much farther. Since it consists of a large wheel rigidly attached to a small wheel (the axle or the shaft), when one part turns the other also does. Some examples of the wheel and axle are a door knob, a water wheel, an egg beater, and the wheels on a wagon, car, or bicycle. When force is applied to the wheel (thereby turning the axle), force is increased and distance and speed are decreased. When it is applied to the axle (turning the wheel), force is decreased and distance and speed are increased.

Inclined plane

An inclined plane is simply a sloping surface. It is used to make it easier to move a weight from a lower to a higher spot. It takes much less effort to push a wheel barrow load slowly up a gently sloping ramp than it does to pick it up and lift it to a higher spot. The trade-off is that the load must be moved a greater distance. Everyday examples are stairs, escalators, ladders, and a ship's plank.

Wedge

A wedge is an inclined plane that moves and is used to increase forceeither to separate something or to hold things together. With a wedge, the object or material remains in place while the wedge moves. A wedge can have a single sloping surface (like a door stop that holds a door tightly in place), or it can have two sloping surfaces or sides (like the wedge that splits a log in two). An axe or knife blade is a wedge, as is a chisel, plow, and even a nail.

Screw

A screw can be considered yet another form of an inclined plane, since it can be thought of as one that is wrapped in a spiral around a cylinder or post. In everyday life, screws are used to hold things together and to lift other things. When it is turned, a screw converts rotary (circular) motion into a forward or backward motion. Every screw has two parts: a body or post around which the inclined plane is twisted, and the thread (the spiraled inclined plane itself). Every screw has a thread, and if you look very closely at it, you will see that the threads form a tiny "ramp"

that runs from the tip to the top. Like nails, screws are used to hold things together, while a drill bit is used to make holes. Other examples of screws are airplane and boat propellers.

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Machines, Simple

Machines, Simple

Levers

Mechanical advantage

Pulleys

Wheel and axle

Inclined planes

Wedges

Screws

Compound machines

Resources

In physics, a simple machine is any device that requires the application of only one force in order to perform work. Work is the product of the force applied and the distance moved due to the force. Most authorities list six kinds of simple machines: levers, pulleys, wheels and axles, inclined planes, wedges, and screws. One can argue, however, that these six machines are not entirely different from each other. Pulleys and wheels and axles, for example, are really special kinds of levers, and wedges and screws are special kinds of inclined planes.

Levers

A lever is a simple machine that consists of a rigid bar supported at one point, known as the fulcrum. A force called the effort force is applied at one point on the lever in order to move an object, known as the resistance force, located at some other point on the lever. A common example of the lever is the crow bar used to move a heavy object such as a rock. To use the crow bar, one end is placed under the bar, which is supported at some point (the fulcrum) close to the rock. A person then applies a force at the opposite end of the crow bar to lift the rock. A lever of the type described here is a first-class lever because the fulcrum is placed between the applied force (the effort force) and the object to be moved (the resistance force).

The effectiveness of the lever as a machine depends on two factors: the forces applied at each end and the distance of each force from the fulcrum. The farther a person stands from the fulcrum, the more his or her force on the lever is magnified. Suppose that the rock to be lifted is only one foot from the fulcrum and the person trying to lift the rock stands 2 yd (1.8 m) from the fulcrum. Then, the persons force is magnified by a factor of six. If he or she pushes down with a force of 30 lb (13.5 kg), the object that is lifted can be as heavy as 180 (6 x 30) lb (81 kg).

Two other types of levers exist. In one, called a second-class lever, the resistance force lies between the

effort force and the fulcrum. A nutcracker is an example of a second-class lever. The fulcrum in the nutcracker is at one end, where the two metal rods of the device are hinged together. The effort force is applied at the opposite ends of the rods, and the resistance force, the nut to be cracked open, lies in the middle.

In a third-class lever, the effort force lies between the resistance force and the fulcrum. Some kinds of garden tools are examples of third-class levers. When a person uses a shovel, for example, one holds the handle end steady to act as the fulcrum, while using the other hand to pull up on a load of dirt. The second hand is the effort force, and the dirt being picked up is the resistance force. The effort applied by the second hand lies between the resistance force (the dirt) and the fulcrum (the first hand).

Mechanical advantage

The term mechanical advantage is used to described how effectively a simple machine works. Mechanical advantage is defined as the resistance force moved divided by the effort force used. In the lever example above, for example, a person pushing with a force of 30 lb (13.5 kg) was able to move an object that weighed 180 lb (81 kg). So, the mechanical advantage of the lever in that example was 180 lb divided by 30 lb, or 6.

The mechanical advantage described here is really the theoretical mechanical advantage of a machine. In actual practice, the mechanical advantage is always less than what a person might calculate. The main reason for this difference is resistance. When a person does work with a machine, there is always some resistance to that work. For example, a mathematician can calculate the theoretical mechanical advantage of a screw (a kind of simple machine) that is being forced into a piece of wood by a screwdriver. The actual mechanical advantage is much less than what is calculated because friction must be overcome in driving the screw into the wood.

Sometimes the mechanical advantage of a machine is less than one. That is, a person has to put in more force than the machine can move. Class three levers are examples of such machines. A person exerts more force on a class three lever than the lever can move. The purpose of a class three lever, therefore, is not to magnify the amount of force that can be moved, but to magnify the distance the force is being moved.

As an example of this kind of lever, imagine a person who is fishing with a long fishing rod. The person will exert a much larger force to take a fish out of the water than the fish itself weighs. The advantage of the fishing pole, however, is that it moves the fish a large distance, from the water to the boat or the shore.

Pulleys

A pulley is a simple machine consisting of a grooved wheel through which a rope runs. The pulley can be thought of as a kind of lever if one thinks of the grooved wheel as the fulcrum of the lever. Then the effort force is the force applied on one end of the pulley rope, and the resistance force is the weight that is lifted at the opposite end of the pulley rope.

In the simplest form of a pulley, the grooved wheel is attached to some immovable object, such as a ceiling or beam. When a person pulls down on one end of the pulley rope, an object at the opposite end of the rope is raised. In a fixed pulley of this design, the mechanical advantage is one. That is, a person can lift a weight equal to the force applied. The advantage of the pulley is one of direction. An object can be made to move upward or downward with such a pulley. Venetian blinds are a simple example of the fixed pulley.

In a movable pulley, one end of the pulley rope is attached to a stationary object (such as a ceiling or beam), and the grooved wheel is free to move along the rope. When a person lifts on the free end of the rope, the grooved wheel and any attached weight slides upward on the rope. The mechanical advantage of this kind of pulley is two. That is, a person can lift twice as much weight as the force applied on the free end of the pulley rope.

More complex pulley systems can also be designed. For example, one grooved wheel can be attached to a stationary object, and a second movable pulley can be attached to the pulley rope. When a person pulls on the free end of the pulley rope, a weight attached to the movable pulley can be moved upward with a mechanical advantage of two. In general, in more complicated pulley systems, the mechanical advantage of the pulley is equal to the number of ropes that hold up the weight to be lifted. Combinations of fixed and movable pulleys are also known as a block and tackle. Some blocks and tackles have mechanical advantages high enough to allow a single person to lift weights as heavy as that of an automobile.

Wheel and axle

A second variation of the lever is the simple machine known as a wheel and axle. A wheel and axle consists of two circular pieces of different sizes attached to each other. The larger circular piece is the wheel in the system, and the smaller circular piece is the axle. One of the circular pieces can be considered as the effort arm of the lever and the second, the resistance arm. The place at which the two pieces is joined is the fulcrum of the system.

Some examples of the wheel and axle include a door knob, a screwdriver, an egg beater, a water wheel, the steering wheel of an automobile, and the crank used to raise a bucket of water from a well. When the wheel in a wheel and axle machine is turned, so is the axle, and vice versa. For example, when someone turn the handle of a screwdriver, the edge that fits into the screw head turns at the same time.

The mechanical advantage of a wheel and axle machine can be found by dividing the radius of the wheel by the radius of the axle. For example, suppose that the crank on a water well turns through a radius of 2 ft (61 cm) and the radius of the axle around which the rope is wrapped is 4 in (10 cm). Then, the mechanical advantage of this wheel and axle system is 2 ft divided by 4 in, or 6.

Inclined planes

An inclined plane is any sloping surface. Many people have used an inclined plane at one time or another when they tried to push a wheelbarrow or a dolly up a sloping board into a truck. One major difference between an inclined plane and a lever is that motion always takes place with the latter, but not with the former.

The primary advantage of using an inclined plane is that it takes less effort to push an object up an inclined plane than it does to lift the same object through the same vertical difference. Just compare how difficult it might be to lift a can that weighs 10 lb (4.5 kg) straight up into a truck compared to how difficult it would be to push the same can up a sloping board into the truck.

The mechanical advantage of an inclined plane can be found by dividing the length of the plane by its height. In the preceding example, suppose that the sloping board is 10 ft (3m) long and 2ft (61 cm) high. Then, the mechanical advantage of the inclined plane would be 10/2, or 5. A person could move the ten pound weight into the truck using a force only one-fifth as great as if the can were lifted directly into the truck.

Wedges

A wedge is an inclined plane that can be moved. Chisels, knives, hatches, carpenters planes, and axes are all examples of a wedge. Wedges can have only one sloping plane, as in a carpenters plane, or they can have two, as in a knife blade. The mechanical advantage of the wedge is calculated in the same way as with an inclined plane by dividing the length of the wedge by its width at the thickest edge.

Screws

A screw can be considered to be an inclined plane that has been wrapped around some central axis. A person can see this relationship by making an inclined plane out of paper and, then, wrapping the paper around a pencil. The spiral shaped form that you make is a screw.

Screws can be used in two major ways. First, they can be used to hold things together. Some simple examples include wood and metal screws and the screws on jars and bottles and their tops. Screws can also be used to apply force on objects. The screws found in vises, presses, clamps, monkey wrenches, brace and bits, and corkscrews are some examples of this application.

The screw acts as a simple machine when an effort force is applied to the larger circumference of the screw. For example, a person might apply the effort force to a wood screw by turning a screwdriver. That force is then transmitted down the spiral part of the screw called the thread to the tip of the screw. The movement of the screw tip into the wood is the resistance force in this machine. Each complete turn of the screwdriver produces a movement of only one thread of the screw tip into the wood. This distance between two adjacent threads is called the pitch.

The mechanical advantage of a screw can be found by dividing the circumference of the screw by its pitch. For example, suppose that a carpenter is working with screws whose heads have a circumference of 1 in (2.54 cm)

KEY TERMS

Compound machine A machine consisting of two or more simple machines.

Effort force The force applied to a machine.

Friction A force caused by the movement of an object through liquid, gas, or against a second object that works to oppose the first objects movement.

Mechanical advantage A mathematical measure of the amount by which a machine magnifies the force put into the machine.

Resistance force The force exerted by a machine.

and a pitch of 1/8 in (.33 cm). Then the mechanical advantage of these screws is 1 divided by 1/8, or 8. The carpenter magnifies his or her efforts by a factor of 8 in driving the screw into a piece of wood.

Compound machines

In many instances, the combination of two or more simple machines achieves results that cannot be achieved by a simple machine alone. Such combinations are known as compound machines. An example of a compound machine is the common garden hoe. The handle of the hoe is a lever, while the blade that cuts into the ground is a wedge. Machines with many simple machines combined with each othersuch as typewriters, bicycles, and automobilesare sometimes referred to as complex machines.

Resources

BOOKS

Editors of Delta Education. Simple Machines. Nashua, NH: Delta Education, 2003.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.

David E. Newton

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