How to convert kn/m to n/mm?

When force is applied to objects in a certain way, they rotate. This inclination of objects to rotate under the influence of a force is torque. Torque depends on the force and the distance between the axis of rotation of the object and the place where the force producing the rotation is applied. The force here is a vector, therefore even if its magnitude stays the same, it changes with the change of the angle between the direction at which the force is applied and the lever. In particular, if the force acting upon the lever is perpendicular to the lever, the torque is the strongest, and it decreases to zero as the direction of the force aligns with the lever. In essence, torque represents what combination of the magnitude of force and distance is needed and in what direction force needs to be applied to produce a given amount of rotation.

You can see this in the illustration. Here the forces marked as F2, F3, and F5 are perpendicular to the line, which connects the point of the application of force and the center of the helm. They produce maximum torque. Forces F1 and F4 are not perpendicular to the line connecting the point of force application and the center of rotation, and because of this, the torque is reduced.

When performing a specific task that involves rotating an object using force, one would need a given amount of torque. Because the resulting torque is affected by the magnitude and the direction of the force as well as the distance from the axis of rotation to the point of application of force, one can manipulate either the force or the distance to attain a certain amount of torque. This property has been used by people for thousands of years.

Generally, it is easier to increase the distance between the object and the point where the force is applied, than to increase the force, so when human or animal power is not sufficient to complete a given task that involves rotation, people have been increasing the distance, often by using levers and other devices, to increase the torque. For example, to grind flour at the mill or to lift a heavy bridge, people or animals rotated devices with long handles around their axis and increased the human or animal power by the coefficient, equal to the increase in the distance between the axis of the rotating object and the point of force application.

Another example of manipulating torque is in bicycle pedals. The further we place our feet away from the center of the bicycle’s wheel, the easier it is to rotate it by using the pedals. The length of our own legs is limited, therefore the pedal length cannot extend beyond a certain length, but pedals still make it easier to move the bicycle. Some people, especially in developing countries where some modern technology is not freely available or is expensive, modify bicycle pedals, wheels, or entire frames with two wheels to make hand-operated machines. One example is of making a hand-operated wheelchair from the bicycle pedals and recycled wheelchair parts. In this case, the pedals could be slightly extended to allow for better torque, although depending on the design such extensions may make the operation of the wheelchair less convenient.

We also use a wrench to increase torque. The design of a wrench allows a good grip for nuts and bolts and has a long handle to magnify the force applied with the wrench. Some jobs require only a small wrench, but to turn a bolt that is really stuck, for example, if it rusted, a wrench with a longer handle is better, because it increases torque. If no wrench is available, it is possible to use pliers instead. Their long handles produce the same effect as the handles of a wrench, although they may offer less grip and can damage a nut or bolt head.

A wrench is designed in such a way that if the right size is chosen, no additional force is necessary for gripping. When using pliers, however, one needs to apply force to bring the two handles closer together and grip the object, in addition to the force needed to rotate this object. Therefore wrenches are more energy-effective for many applications. In some cases, pliers are better, however, because they allow one to vary the size of the object being gripped. They can also more easily be used at an angle. Applying force at an angle may decrease the torque, but it is useful in situations when the object being rotated is hard to reach.

Rubber grip tools that help with opening tightly closed jars are similar to wrenches. The rubber grip is not related to torque, it simply prevents the tool from slipping off the lid. The handle does increase torque, however. The longer this handle — the more our initial force is magnified.

A flywheel is a good example of a device that uses torque to generate energy, which is then stored within the flywheel for further use. The torque increases the speed at which the wheel rotates and increases the stored energy. When the energy is needed, torque is applied again to slow down the rotation and the energy is released. These devices are useful when the energy supply is not continuous — they can provide energy when the original energy supply dwindles. A vehicle engine is a good example of this. In the engine, the energy released through burning the fuel comes in bursts, and the flywheel collects it and ensures a constant supply.

In some cases the opposite is necessary. Flywheels also allow releasing an amount of energy larger, than the original source can provide. In this case, the energy is stored gradually and then released in a burst, when needed.

When two people sit on a seesaw, their weight is the force that makes the seesaw move up and down, by partially rotating about its center. Children of the same weight can play on the seesaw easily if they sit roughly the same distance away from the fulcrum. It is not so easy for children, whose weight differs significantly, because the heavier child would bring the seesaw down and the lighter child up. In this case, the lighter child would not be able to push the seesaw back down. This is because the bigger child produces more torque. To counter this, the heavier child could balance torque with the lighter child by moving closer to the center of the seesaw. For example, the bigger child who is three times heavier than the smaller child should sit three times closer to the center of the seesaw to ensure balance.

The levers operate on a similar principle: torque plays a role in helping reduce the amount of force needed to perform a given task. Generally, a lever is a long object, like a plank or a handle, that rotates about a point called fulcrum. A force is applied to the lever at a specific point, and it is then either magnified or minimized, depending on the construction of the lever and on the needs of the person, using the lever.

There are three types of levers, depending on where the force is applied, where the output force is directed, and where the fulcrum is located. Usually, they are referred to as class one, class two, and class three levers. Often the force applied to the lever or the input force is called the effort, while the output force is often referred to as the resistance. This word is chosen because indeed, the output force resists the effort. For example, if you try to lift a load using a lever, the weight of the load will resist the input force or the effort, but if the effort is strong enough, then the resulting force will produce the work required. Our own bodies, as well as bodies of other animals also use the same principles and operate some body parts as levers, to minimize the energy needed to perform certain tasks, as we will show in examples below.

Class one levers are similar to seesaws in their construction. The fulcrum is located in the middle. The effort is on one end, while the resistance is on the other end. The fulcrum for the levers in class two is located on one end of the lever, the effort is applied at the opposite end, and the resistance is close to the fulcrum, with the direction, opposite of the effort. The design of class three levers is the opposite of the construction of class two levers. The fulcrum is still on one end of the lever, but the force closest to it is the effort, while the force on the other edge, acting in the opposite direction of the effort, is the resistance.

Some scales, balanced in the middle, operate as class one levers. Scissors are a combination of two class one levers; they allow us to cut thick materials that may be difficult to cut with a knife, for example. The length of the handles allows decreasing the magnitude of the force, necessary for cutting. Conversely, placing the object to be cut further away from the pivot, which is the fulcrum, makes it more difficult to cut.

Scissors or shears, meant to cut thicker and harder materials like branches or sheet metal, often have longer handles to increase the torque. In some cases, a spring is added to the design for mechanical advantage. Some specialized scissors have additional features. For example, trauma shears, meant to cut clothes away from the body of an injured person, have blades with rounded edges, to prevent injury to the skin. Other scissors intended for use in the medical profession can have curved or sharp edges, depending on the intended use, and some are small enough to allow the surgeon to work with delicate tissue, while still having a mechanical advantage over other cutting instruments like knives. Scissors are sometimes even used in eye surgery and can be as small as 6 cm long, with blades 2 cm long and shorter.

A crowbar is another tool classified as a class one lever, although it could be used as a class two or class three lever as well. Often it is used to remove nails or to pry apart two connected elements. It can also be used to lift heavy objects, especially on small heights. A crowbar is notorious for being used for burglary, although creative criminals would use any tool that would do the job.

In our bodies, the mechanics that operate the movement of a human head, as well as heads of many other animals, is an example of class one lever. The head is balanced at the neck, which becomes a fulcrum. The effort is applied by the muscles on one side of the head, and the resistance is applied on the opposite side. When enough force is applied, the head tilts in the direction of the output force, or the resistance.

Our mouth, when used for chewing, as well as beaks of birds, are examples of class two levers. So are nutcrackers. Nutcrackers can be made of metal or wood, and these days are often ornamental. In some cases, they are used solely for decoration, as the wooden nutcrackers in the shape of soldiers, kings, and other characters, popular in North America for displaying around Christmas time, as seasonal decorations. Some believe that the decorative nutcrackers shaped as figurines originated in Germany, where it is still a popular local craft. In the German countryside, they are often made to sell to tourists. These days it is more common to use simple and functional designs for cracking nuts and hard shells of lobsters and crabs. The claws of lobsters and crabs are actually class two levers as well, working on a similar principle as nutcrackers.

A garlic press is also similar in design to these devices and is a second class lever as well. Not all chefs agree to use it, because some believe that the taste of pressed garlic is inferior, but others prefer pressed garlic for its more intense flavor, and for the ease of pressing it.

Our feet, as well as the feet of some animals, also act as second class levers. The fulcrum is around the toes, and the muscles apply effort around the heel. Our weight acts as resistance. This “lever” helps us balance on our feet, raise and bring down our body.

Other examples of second class levers are wheelbarrows, brakes in vehicles, and doors, among others. If we push a door near the fulcrum, it would be nearly impossible to open it, but we can easily swing it when applying force on the side, opposite from the hinges. This is the reason for placing door handles on the side, opposite the hinges. To increase torque and to reduce the force, needed to open it, the heavy door can be made wider.

Bottle openers also act as class two levers, in particular the stand-alone tools, not the ones attached to a wall or another surface. Some of these openers are tools included in pocket knives, and some are small and can be carried on a keychain. Sometimes a bottle opener can act as a class one lever, if placing the opener differently and pushing down on it, instead of up.

Our arms, when used to lift heavy objects, work as third-class levers. So do our legs when we push off the ground while walking or running. In this case, the knees and elbows act as fulcrums. Similarly, when we “extend” our arms with tools, such as baseball bats or tennis rackets, we create additional class three levers. We apply the effort near the fulcrum to pivot these items, and the resistance force is at the end of the bat or racket, where they come in contact with the ball. Similarly, a fishing rod is also a class three lever, because we apply the force to pivot it around the grip area.

A hammer is another example of a class three lever, and so are other similar tools, including shovels, rakes, brooms, and fly swatters. Tools like staplers, tongs, and tweezers are class three levers as well, but consist of a double lever, with one acting towards the other.

Now, let us look at an example of using a lever. Imagine that an average person can normally lift a stone of about 20 kg. It is strenuous, but possible to do, even for a person that is not very strong. On the other hand, a small child would not be likely to lift such a stone. However, if using a crowbar, that is strong enough so it would not break, and long enough, the weight can be reduced considerably, allowing even a child to lift this stone. Famously Archimedes claimed that he could move the Earth if he could have a place to stand on that was far enough away — this claim is based on the same principle. After we lift this heavy stone using class one levers, we can then place it on a class two lever, a wheelbarrow, and transport it, by lifting the wheelbarrow by hands and arms, which are third-class levers.

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