Why do we need levers

Scissors and shears are first-class levers, even though the fulcrum is slightly off center. The centralized fulcrum still serves as the pivot point that allows you to raise and lower the dual bars at one end with the handles on the other end.

Scissors are an example of a lever that uses force to cut or separate materials. As curriculum developer and educator, Kristine Tucker has enjoyed the plethora of English assignments she's read and graded! Her experiences as vice-president of an energy consulting firm have given her the opportunity to explore business writing and HR.

Tucker has a BA and holds Ohio teaching credentials. Levers Used in Everyday Life. Ten Different Types of Levers. A List of Simple Machines. How to Convert Torque to Force. What Are the Different Classes of Levers? A Lesson to Introduce Simple Machines. Mechanism Description of a Manual Can Opener. Today we are ready to learn about three more simple machines.

These include the lever, pulley, and wheel-and-axle. These machines may sound unfamiliar initially, but it is likely you will recognize them when we reveal the many everyday applications, equipment, and appliances in which they are found. Although one of the six simple machines is not superior to another, each machine offers its own distinct advantages for various engineering applications. These advantages, along with how engineers use them, will be discussed in today's lesson as we study these next three exceptional machines.

Following the lesson, students can employ their knowledge alongside their creativity in the hands-on associated activity Machines and Tools, Part II. Many engineers today, especially mechanical engineers, are interested in simple machines and their ability to carry out an immense amount of work with minimal effort. To understand how this is achieved, it is necessary to recall that work is done by applying force to a load and transporting it over some distance.

The more force applied and the further the load is moved, the more work is done. This idea is expressed mathematically as.

We know that a specific amount of work needs to be completed for a certain task. However, nature does not specify exactly how this work may be accomplished. This enables engineers to complete the same amount of work with less force by simply moving the load over a greater distance. This tactic of making work easier is performed with simple machines such as the lever, pulley, and wheel-and-axle.

Figure 2. Three Classes of levers. The lever is the most familiar of all the simple machines because of its plain construction and extensive use in numerous engineering devices. It merely consists of a rigid beam or rod which freely rotates about a fixed point, also referred to as the fulcrum.

By positioning the fulcrum close to a heavy object and applying an effort from far away, levers can be used to lift enormous loads with ease refer to Figure 1. The object being moved by the lever is often called the load , or output force, while the force applied to the lever is called the effort , or input force.

The crowbar is a classic example of how the lever is employed to do work easier. With the crowbar, carpenters can easily extract nails from wood that would be nearly impossible and extremely inefficient without such a handy machine.

Figure 3. A wheelbarrow, a type of second-class lever and one of the six simple machines. Immediately you will see that there is always a fulcrum, load and effort positioned somewhere on the lever, yet it may be difficult to notice how the position of each of these relative to one another can change the characteristics of the lever altogether. For this reason, levers are classified into three different types; called first-, second- and third-class levers see Figure 2.

The classification of each depends on the position of the fulcrum relative to the effort and load. In a first-class lever, the fulcrum is placed between the effort and load to resemble a seesaw. Examples of this type of lever include a balance scale, crowbar, and a pair of scissors. A second-class lever is when the load is placed between the fulcrum and effort.

This lever type has been used in the design of many devices such as a wheelbarrow, nutcracker, bottle opener, and conventional door. Lastly, third-class levers operate with the effort applied between the fulcrum and load. These levers can be found in tweezers, fishing rods, hammers, boat oars, and rakes. Figure 4. A pulley, one of the six simple machines. Throughout history, engineers have found the pulley to be the machine of choice when lifting heavy objects over a direct vertical path.

The pulley is basically a grooved circular disk which acts to guide a rope or cable pulled around its perimeter, as illustrated in Figure 4. With a single pulley, engineers can change the direction of an applied force; such as pulling a rope down to lift a weight up.

However, using a combination of pulleys in a pulley system can change both the amount and direction of the applied effort. To increase the pulley's lifting power, pulley wheels are added to a pulley system so that the effort required to lift objects vertically is largely reduced.

This machine is incorporated into the design of various engineering systems such as a crane, where huge loads are manipulated with a little force supplied by a relatively small motor. Some cranes can have numerous pulley wheels and a complex array of cables so that the ability to lift heavier objects is even greater. Many other devices employ the pulley in order to benefit from its amazing potential, including an elevator, sailboat, and a basic flagpole. The last simple machine we are going to learn about is the wheel-and-axle, which engineers primarily use to increase a turning or rotational force.

This device is composed of a circular wheel directly connected to a circular shaft or axle, and turned to rotate about a common axis see Figure 5.

From this arrangement, you may notice how the wheel and axle operates similar to that of the lever; however, it is different in the sense that it has the ability to increase a rotational force instead of a linear force. Engineers commonly refer to a rotational force as torque. In order to remain consistent with the definition of mechanical advantage, we define the wheel and axle such that the effort or input force is always applied to the wheel and the load or output force is always acting on the axle.

Figure 5. A wheel-and-axle, one of the six simple machines. In most cases, the axle is smaller than the wheel and the applied torque is magnified by the machine; however, this configuration is not always the case. In some instances, the axle is larger than the wheel, and the input distance is increased by the machine instead of the input torque.

Examples of the wheel and axle include a screw driver, steering wheel, jet engine, mechanical gears, and even doorknobs. A bicycle is a great example of several simple machines like the wheel-and-axle, lever, and pulley, integrated into one device see Figure 6. The front and back tires are wheel-and-axles, where the tires rotate around the axle in the center where the gears are fixed.

The gears and chain act as a pulley and help to drive the wheel on its axle. There are several levers on a bicycle, one of which is the pedal. All three of these simple machines are necessary for a bicycle to move! As you ride your bike, your leg transfers energy to the pedal lever , which then gets tranferred from the pedal to the chain and gears pulley system.

This energy finally gets transferred to the wheel-and-axle system tires and then to the ground to make the bicycle move forward! Figure 6. A bicycle, an example of a wheel-and-axle simple machine. The mechanical advantage of a machine characterizes its ability to do work efficiently and effectively. Therefore, anytime a simple machine is considered for an appropriate engineering system, it is necessary to determine its associated mechanical advantage.

Note: The "wheel and axle" should not be confused with the kind you find on a wagon The wheel the one you roll is just a device designed to reduce friction. The "wheel and axle" is something you turn to gain a mechanical advantage.

A common winch see image below is an example of a "wheel and axle". Using only one person, you can draw a boat snug to the top of the trailer with this device and you also have wheels which the boat rides on during this process to reduce friction. If the counter-weight matches the load A special kind of pulley, called an eccentric pulley, does give you a mechanical advantage because the center of rotation is not at the geometric center of the pulley. This can be found on a compound bow.

If you have ever used a compound bow, you will find that initially, the bow is very difficult to pull back, but once pulled back far enough, it becomes quite easy to hold and steady. Why do we mention this kind of pulley here in a section on levers?

This is because the eccentric pulley is a first class lever in disguise. At first, when you initially draw the arrow, you would normally expect the easiest effort, but it is actually quite difficult. The eccentric pulley produces a mechanical dis advantage in this position and you really have to strain at first. However, once you pull the arrow back and expect the greatest struggle with the bow This is because the pulley pivots around giving you a mechanical advantage.

The Compound Bow - can you see that the eccentric pulley is really just a lever? Power is nothing more than the rate at which work is done how fast energy is being used. Common units are the watt and horsepower. The concept of power was covered in a previous unit, but needs our attention again because of how it is related to torque. A car engine can only produce so much power. This power can be delivered to the wheels of the car in one of two ways In fact, the formula looks like this:.

This means that the power output of an engine must be managed to either produce a high torque at low speed or low torque at a high speed. This is where a car's transmission comes in. It is designed to deliver the proper torque and speed to the wheels using systems of gears. It is much simpler to use an ordinary bicycle to illustrate the point. In this case, you are the device providing the output power to the wheels.

The bike has a chain that transfers the power from the pedals to the back wheel. If there are no changeable gears on the bike Also, you have to pedal harder if you start going up a hill need more torque at the wheels.

Gears let you exert yourself about the same, but allows you to trade speed for torque or vice versa. When the chain by the pedal is set on the smallest gear and the rear axle wheel is set on the largest gear, you are trading speed for torque. You will go very slow, but provide a high torque at the wheels for going up a hill.

When going down a hill, you need very little torque at the wheels so you reverse the gears and get lots of speed. The same thing happens with your car. The engine's power output can be fairly constant but the transmission plays the same torque vs. If you want to go up a steep hill, you need more torque at the wheels so you need to sacrifice speed RPM's to accomplish the task.

Question 1 - The effort force must be applied as far from fulcrum as the load is from the fulcrum. That is, symmetry rules, so the right side is a mirror image of the left side. Question 2 - The MA is 1. No advantage, no disadvantage. Question 3 - The effort force must be applied farther from the fulcrum than the load is from the fulcrum.

Question 5 - In some cases you are more interested in freedom of motion rather than force multiplication. In this case, you provide a large effort force over a short distance to provide a smaller force over a larger distance. Many hydraulic systems need to apply this type of lever because the output piston doesn't move very far.

You will likely see some 1st class levers where the hydraulics act as the effort force, but offer a wide range of output motion. This animation shows 3 levers which all produce a mechanical dis advantage. Question 6 - a.