Robert Krampf's Friction Experiment

This Week's Experiment - #268 Science Friction

Greetings! Well, I finished up the shows in the Ft. Myers area and for the next couple of weeks, I will be visiting schools in the area around Titusville, FL. I always look forward to shows in this area. The schools are good and seem to have a strong science program. Also, many of the schools are also near the beach, which means I can take some nice walks between my morning and my afternoon shows. In addition to the birds and shells, I just got a new microscope and am eager to try it out.

This week's experiment is a result of an e-mail from Karen H. She asked about past experiments dealing with friction. As I began looking, I discovered that I had been ignoring this interesting subject. This week I will correct that. You will need:

bulleta rubber band 
a shoe
a ruler

Friction is a subject that I thought I knew a lot about, until I began to really look at it. Then I realized that the subject was a lot more complex than just observing how things rub together. Tribologists (scientists that study friction) divide friction into static and kinetic. To see the difference, cut a rubberband, so that you have a long string of rubber. Tie one end to a shoe. Set the shoe on the floor. Grasp the other end of the rubberband and begin pulling. Pull gently at first and then pull harder and harder until the shoe begins to move. Use the ruler to measure how long the rubberband stretches before the shoe begins to move.

Before it begins to move, it is experiencing static friction between the shoe and the floor.

Once the shoe starts moving, measure how far the rubberband is stretched as you keep it moving. You will probably notice that it is not stretched as far as when you were trying to get it started. When it was sitting still, you had to overcome the static friction (as well as inertia) to get it going.

Once it was going, you had to contend with kinetic friction, which is less that static friction, if everything else stays the same. In other words, once you get the thing moving, there is less friction to resist its movement.

How can we change the amount of friction? You could try using a different surface. Spread some aluminum foil on the floor and place the shoe on top.

Try pulling the rubberband again and measure the static and kinetic friction.

Did the shoe slide across the foil, or did the foil slide across the floor with the shoe? One way, you are measuring the friction between shoe and foil. The other you are measuring the friction between foil and floor. Try it both ways and compare.

Add some weight by putting some rocks into the shoe. Try again and see how increased weight changes the friction.

Try adding some roughness by pouring some salt or sugar under the shoe, or placing it on a sheet of sand paper.

Tape the foil to the floor and rub it with a little cooking oil. Then try the shoe again, measuring static and kinetic friction. What did you notice? Does this give you an idea of why they use oil as a lubricant in cars and other motors? Can you think of why the layer of oil would change the friction?

Kind of material, weight, roughness and lubricants can all have an impact on the amount of friction. Of course, tribologists argue about these all the time. Some say that roughness is different from friction. Others say that friction is a result of chemical attractions. Some even say that friction is due to conversion of energy into sound waves, which explains why dragging something heavy always seems to make such a terrible screech. It is interesting that even something as common as friction can still cause professors to argue over how it works. It is nice to know that there are still so many mysteries in science, and anything that can make teachers argue among themselves is a good thing.

****************************************

This weekly e-mail list is free from charge. You are welcome to forward it to friends, print it in your newsletter, repost it on the Internet, etc., as long as you do not charge for them and my name and e-mail address are included.

Please forward this e-mail to anyone that you think might enjoy it.

To join the list, send a blank e-mail to krampf-subscribe@yahoogroups.com

To unsubscribe, send a blank e-mail to krampf-unsubscribe@yahoogroups.com

An on-line archive is located at

http://groups.yahoo.com/group/krampf/messages

***************************

Robert Krampf's on-the-road schedule

My calendar is now on-line. You can see a detailed calendar, with dates, times, schools, etc. at http://www.krampf.com/mainsubjectsfolder/m_tour.html

***************

The Experiment of the Week reaches over 160,000 people in the following countries. If your country is not listed, let me know. Antigua Argentina Australia Austria Azerbaijan Bahamas Bahrain Belgium Belize Bermuda Brazil Cameroon Canada Central African Republic China Colombia Costa Rica Cuba Cyprus

Ecuador Egypt El Salvador England Finland France Germany Greece Grenada Guatemala Haiti Holland Honduras Hong Kong Hungary Iceland India Indonesia Iran Iraq Ireland Israel Italy Jamaica Japan Jordan Kenya Republic of Korea Kuwait Latvia Lebanon Lithuania Malaysia Mali Marshall Islands Mexico Moldova Mongolia Morocco Namibia Netherlands Newfoundland New Zealand Nicaragua Nigeria Norway Oman Pakistan Palestine Papua New Guinea Paraguay Peru Philippines Poland Portugal Puerto Rico Rwanda Romania Russia Saudi Arabia Scotland Senegal Singapore South Africa Spain Sri Lanka Suriname Swaziland Sweden Syria Taiwan Thailand Togo Trinidad Turkey Ukraine United Arab Emirates United Kingdom United States of America Venezuela Wales Zambia Zimbabwe

***************************

Check out my web site at http://www.krampf.com

From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381