GRADES K-8
SIMPLE MACHINES - ROTATING WHEELS

BACKGROUND INFORMATION: What Is A Machine? Think of the word machine and you may conjure up (imagine) pictures of big bulky cranes, bulldozers, or even a copying machine. However, in science and engineering the word "machine" has a very specific meaning. A machine is any device that can apply mechanical energy at one point and deliver it in a more useful form at another point. A machine is any device that provides a "mechanical advantage". Simple machines are all around us and help us accomplish work faster and easier many times each day. Let's use some examples to explain the concept of simple machines and mechanical advantage. You have a box with 120 pounds of books to return to the library. A friend will drive you to the library in their truck. You just have to get the books into the bed of the truck. Wow! That would be a lot of weight to lift using your own muscles. You need a mechanical advantage! Instead of carrying the box of books to the truck you place them in a wagon or on a dolly and pull the wagon (or dolly) to the truck. The rotating wheels on these simple machines make it easier to move the books. Next instead of lifting the books from the ground to the truck, you make a ramp out of a board and push the box up the ramp into truck. The ramp is a simple machine called an inclined plane. Now you are ready for a soda! When you use a bottle opener to pry the cap off of a soda bottle or the pop top on a soda can, you have used a simple machine. The bottle opener and pop tops are forms of another type of simple machine called a lever. How Do Machines Help Us? There are many different types of machines with varying capabilities and functions. Different machines may: Transform energy (change from one type of energy to another type). Example: a generator can change mechanical energy to electrical energy; a steam turbine changes thermal (heat) energy to mechanical energy. Transfer (deliver) energy. Example: an automobile's drive train transfers energy to the rear wheels. Increase or multiply force. Example: pulleys can lift more weight with less force. Increase or multiply the speed. Example: as you pedal a bicycle you turn the bicycle's gears, however the bicycle wheels move faster than the gears. Changes the direction of the force. Example: you pull a flag pole pulley down to raise the flag. Reduce friction. Common simple machines include the: lever inclined plane wheel and axle wedge pulley screw gear and belt cam and crank spring rotating wheel Simple machines are typically used when: The amount (the magnitude) of the force needed can not be applied without the aid of a machine. This applies to our example of the 120 pounds of books. The books were too heavy to move without the aid of two simple machines - a wagon with rotating wheels and an inclined plane. The direction of applied force is not in the desired direction. For example, when you pull down on the flag pole rope you want the flag to move in the opposite direction; you want the flag to go up, not down. A pulley is a simple machine that allows you to change direction of a force. Mechanical Advantage Archimedes was a great mathematician and engineer who was born in 287 BC in Syracuse, Sicily. He is credited with the development of many of our modern day mathematical and mechanical principles (such as Archimedes' principle, the concept of pi (p) and geometric proofs) and machines like the lever, pump, and pulleys. Archimedes understood the concept of mechanical advantage and how to use it to move or lift heavy objects with less force. In mathematical terms the mechanical advantage (abbreviated as MA) is the ratio of the load to the applied force. MA = Load/Applied Force A good mechanical advantage is greater than 1. The greater the mechanical advantage the smaller the applied force needed to accomplish the task. Although machines use less force to accomplish the same task there is a trade-off. Using the ramp required less force than lifting those 120 pounds of books from the ground to the bed of the truck. Although less force was needed to move the box of books into the truck, that reduced force must be applied for a longer period of time or longer distance. Rotating Wheels In the activity, you lifted the books up, pushed them across a table and placed them in a wagon and pushed them across the table. What mechanical advantages did you detect? It took less force and muscle strength on your part to place the books in the wagon and then push it. Additionally, the wagon's wheels did not encounter as much friction. As you pushed the books along the table the books and the table created a resistance. However, the wheels of the wagon had less surface in contact with the table reducing that friction. It is easy to spot the simple machine, the rotating wheel, in our daily lives. Some applications of rotating wheels include: Bicycle Car Skate Board Inline or Roller Skates Moving Dolley REFERENCES: Freeman, Ira M. and Durden, William J., Physics Made Simple, Doubleday, New York, 1990. Halpern, Alvin, Schaum's Outline of Theory and Problems of Beginning Physics I: Mechanics and Heat, McGraw-Hill, Inc., New York, 1995. Macaulay, David, The Way Things Work, Houghton Mifflin Company, Boston, 1988.