After this activity, students should be able to:

• Define and describe friction in everyday life.
• Relate friction to bones and joints in the human body.
• Explain what the human body and engineers do to reduce friction.
• Describe how the astronauts learn to work without friction in outer space.

Does anyone know what friction is? (Take answers from the class.) Friction is a force that is created when two things rub together. Rub your two hands together. Hear the noise that your hands make? That noise is friction. Friction resists motion, and can make it hard to push one object over, or next to, another object. Think about sliding on the floor in your socks — is it easier to slide across a tile floor or a carpeted floor? Well, a carpeted floor has more friction, making it harder to slide around in your socks. Friction can lead to wear and tear by slowly wearing material down over time. There are a lot of places that friction occurs: two of those places are in machines and in our bodies.

Here on Earth, friction can cause problems by wearing down machines. For example, when you frequently ride a skateboard, you wear down the wheels of the skateboard and have to replace them (because the wheels constantly roll over cement or other surfaces, causing friction which then causes wear). Do you think friction can wear down the bones in our bodies? It certainly can! The same way wheels wear down after continual use on pavement.

One place that friction affects our bodies most is in our joints. There are many different types of joints in the human body, but today we are going to learn about just three: ball and socket joint, hinge joint and sliding joint. Your elbows and knees are hinge joints; they move in one direction like hinges on a door. Your shoulder is a good example of a ball and socket joint. It can move in all directions. Make a fist with your right hand and put it into your left open hand. Now move it all around. (Note: teacher should demonstrate.) That is how a ball and socket joint works. Sliding joints move in small sliding motions like rubbing your hands together. You can find these in your hands and feet.

These joints all have something in common ─ they are called synovial joints, which means that they have a cavity in the joints that contains synovial fluid. Joints are not just made of bone, because if they were, friction would keep them from moving smoothly. The body uses cartilage and the synovial fluid to reduce friction in joints. Osteoarthritis happens when cartilage wears away over time and exposes bone to bone and increases friction, which is very painful.

In outer space, astronauts have to deal with another problem altogether - the lack of friction! Because there is almost no gravity in outer space, there is no friction either. This means that you can push a very heavy object, and, since it does not have any friction to slow it down, it will just keep going and going! If you kick a soccer ball in space, it does not slow to a stop, it just keeps rolling forever. This lack of friction can make it difficult for astronauts to work on the space station or on space shuttles. Since astronauts are not used to working without friction, they have a special place at NASA where they can experience what a frictionless environment feels like.

This special frictionless environment is called the Precision Air-Bearing Facility (PABF), and it is like a huge air hockey table (see Figure 1). It is a big, smooth, metal floor with very large objects placed on the floor. Air is then pushed out from each of the objects' bottom edges, causing them to hover just a little bit above the floor ─ similar to an air hockey puck. The astronauts practice pushing these large objects around, helping them learn how to guide and direct objects in outer space.

Figure 1. NASA's Precision Air-Bearing Facility (PABF). click for copyright

Engineers can learn a lot about how to design machines by studying the human body. Many machines that engineers have designed use these three types of joints: ball and socket, hinge, and sliding, just like the body does. Engineers also use oil to create a barrier between two surfaces to reduce friction ─ just as the body uses synovial fluid to reduce friction. Biomechanical engineers, who work with helping people who have been injured, can design new bones for the human body and help reduce friction of joints that are rubbing together and causing pain. Today, we are going to learn a little more about friction and how it relates to joints in our bodies. We are going to look at waxed paper and compare that to cartilage. Then, we are going to compare cooking oil to synovial fluid. Let's go!

Before the Activity

With the Students

This activity is messy! Consider covering the tables with newspaper beforehand, and having paper towels ready for spills.

If waxed paper is not available, try using aluminum foil instead.

Remind students to be careful when the vegetable oil is poured onto the waxed paper, as it will run very easily (and quickly) causing even more of a mess!

The Society of Women Engineer's has a great activity in which students create a model arm and shoulder, looking at biomechanics and sports performance. The website for this is: http://www.swe.org/iac/LP/biomed_03.html

Have students research different engineering applications that use lubrication to reduce friction.

Ask students to design another way that NASA could create a frictionless environment in which the astronauts could train for travel to outer space.

Canright, Shelly. National Aeronautics and Space Administration, For Students (Grades 5-8), Features and News, "Astronauts in Training," May 27, 2004, http://www.nasa.gov/audience/forstudents/5-8/features/F_Astronauts_in_Training.html - accessed April 19, 2006.

Jones, J.R. National Aeronautics and Space Administration, Langley Research Center, "NASA Space Vehicle Design Criteria: Lubrication, Friction, and Wear," NASA SP-8063, June 1971, http://trs.nis.nasa.gov/archive/00000099/01/sp8063.pdf - accessed April 19, 2006.

National Aeronautics and Space Administration, Space Operations Missions Directorate, NASAExplores, Express Lessons and Online Resources, "Two-Ton Hockey Pucks," November 12, 2003, http://www.nasaexplores.com/show2_articlea.php?id=03-071 - accessed April 19, 2006.

Parker, Steve. How the Body Works, London: Dorling Kindersley Limited, 1994.

Ressler, Sandy. National Institute of Standards and Technology, Visualization and Usability Group, OVRT Resources for the Humanoid Animation Working Group, http://ovrt.nist.gov/projects/vrml/h-anim/jointInfo.html - accessed April 19, 2006.

University of British Columbia, Department of Zoology, "Al's Website," http://www.zoology.ubc.ca/~biomania/tutorial/bonejt/anc07.htm - accessed April 19, 2006.

U.S. Department of Health and Human Services, National Institutes of Health, National Institute on Aging, Health Information, Publications, "Age Page: Arthritis Advice," May 2005, http://www.nia.nih.gov/HealthInformation/Publications/arthritis.htm - accessed April 19, 2006.