Levers That Lift Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder

Summary Students are introduced to three of the six simple machines used by many engineers: lever, pulley, and wheel-and-axle. In general, engineers use the lever to magnify the force applied to an object, the pulley to lift heavy loads over a vertical path, and the wheel-and-axle to magnify the torque applied to an object. The mechanical advantage of these machines helps determine their ability to make work easier or make work faster.

Keywords simple machine, lever, pulley, wheel-and- axle, mechanical advantage, work

Learning Objectives After this lesson, students should be able to: Explain how the lever, pulley and wheel-and- axle make work easier. Identify how the lever, pulley and wheel-and- axle are used in many familiar engineering systems today. Discuss the mechanical advantage of a lever, pulley and wheel-and-axle.

Pre-Lesson Assessment Work is defined as the product of force and distance. Which one of these things, force or distance, makes work harder to do? (Answer: Force. If a machine can reduce the force required to do work, it is generally easier to apply it over a longer distance.)

Work is defined as the product of force and distance Work is defined as the product of force and distance. Which one of these things, force or distance, makes work fast or slow? (Answer: Distance. If a machine can reduce the distance required to do work, it is generally faster to carryout the work, even though it is harder to do.)

Introduction 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 very 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.

Many engineers today, especially mechanical engineers, are very 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 Work = Force x Distance

We know that a specific amount of work needs to be completed for a specific 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.

Levers

What class lever is a wheelbarrow? Where is the fulcrum?

Pulleys Ideal for vertical lifting. The more loops/pulleys the greater the mechanical advantage. Only as strong as its weakest link.

Wheel-and-Axle Increases or decreases torque (rotational force)

What are some examples of wheel and axel? (Answer: Screw driver, steering wheel, jet engine, mechanical gears, and even doorknobs.)

Post-Introduction Assessment True or False: The lever has four different components. (Answer: False. The lever has three main components; the fulcrum, effort and load. These components can be arranged in three different ways and so the lever has three different classifications.)

True or False: The lever is a better simple machine than either the pulley or wheel and axle. (Answer: False. While the lever may be a better choice for engineers in a specific design, one machine is not superior to another. Every simple machine has its own unique advantages which can be used for different engineering applications.)

Lesson Background The mechanical advantage of a machine characterizes its ability to do work efficiently and effectively.

If MA = 1 (Mechanical Advantage = 1), Implies the machine has no effect on alleviating work, since the output force is exactly the same as the input force. Most simple machines provide a mechanical advantage greater than 1 (MA > 1) so that work is made easier.

Mechanical Advantage Lever

Pulley A pulley system operates on the principle that a load can be lifted easier by pulling on a rope or cable that is wrapped between a supporting structure and a rigid fixture attached to the load itself.

Wheel-and-Axle Before stating the mechanical advantage of a wheel-and-axle, it is extremely important to remember that the effort is always applied to the wheel, while the load always acts to resists the turning motion of the axle.

Wheel radius/Axle Radius

Lesson Closure  All simple machines are characterized by their ability to provide mechanical advantage, which allows engineers to design devices to make work easier and more efficient. Although one machine is not superior to another, each machine provides its own unique and attractive capabilities which are used by engineers for numerous applications.

The lever is capable of quickly increasing either force or distance; the pulley can lift enormous loads over a vertical path; and the wheel-and-axle is used to easily increase an input torque. These three simple machines, combined with the other three (inclined plane, wedge and screw), give engineers a set of extremely valuable tools to effectively carry out work.