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TE Activity: Sliders

In this hands-on activity, students learn about two types of friction — static and kinetic — and the equation that governs them. They also measure the coefficient of static friction experimentally.

Engineers who really understand friction designed a braking system to improve our safety. Have you been in a car when the driver stopped abruptly by slamming on the brakes, but, instead of stopping or skidding, the car started to chatter? That vibration was caused by the anti-lock brake system (ABS). Since engineers know that a non-skidding wheel has more traction than a skidding wheel, they designed a braking system that prevents the brakes from locking up, which can cause a vehicle to slide. By not skidding, the static friction is maximized and the driver can stop the car quickly without loss of control.

Learning Objectives
Materials
Introduction/Motivation
Procedure
Attachments
Safety Issues
Troubleshooting Tips
Assessment
Extensions
Activity Scaling

Grade Level: 7 (6-8)	Group Size: 3
Time Required: 60 minutes	Activity Dependency : None
Expendable Cost Per Group : US$ 5
Keywords: energy, friction, static friction, kinetic friction, coefficient of static friction, sliding

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Colorado Math
- 1. represent, describe, and analyze patterns and relationships using tables, graphs, verbal rules, and standard algebraic notation; (Grades 5 - 8) [2005]
- 1. read and construct displays of data using appropriate techniques (for example, line graphs, circle graphs, scatter plots, box plots, stem-and-leaf plots) and appropriate technology; (Grades 5 - 8) [2005]
- 1. estimate, use, and describe measures of distance, perimeter, area, volume, capacity, weight, mass, and angle comparison; (Grades 5 - 8) [2005]
- 2. construct, use, and explain procedures to compute and estimate with whole numbers, fractions, decimals, and integers; (Grades 5 - 8) [2005]
Colorado Science
- Standard 1: Students understand the processes of scientific investigation and design, conduct, communicate about, and evaluate such investigations. (Grades 0 - 12) [1995]
- Standard 6: Students understand that science involves a particular way of knowing and understand common connections among scientific disciplines. (Grades 0 - 12) [1995]
- 2.2 Students know that energy appears in different forms, and can move (be transferred) and change (be transformed). (Grades 0 - 12) [1995]

Friction is not always a bad thing just because it can waste energy, wear down parts and cause things to heat up. Often, friction can be a reliable ally. For example, we depend on friction to keep our shoes from sliding out from under us and keep our cars on the road. When friction is removed from these situations, like when there is ice on the road, disastrous things can occur.

There are two types of friction: static friction and kinetic friction. Static friction resists an object to start moving or sliding, which is a good thing when you start walking. If static friction didn't exist, it would be like you were constantly walking on ice! Kinetic friction resists an object that is already moving or sliding and always acts in a direction opposite of the motion. Kinetic friction is the reason that anything freely sliding will eventually come to a stop. It is important to note that static friction is always stronger than kinetic friction.

For flat and horizontal surfaces, both static and kinetic friction between an object and the ground can be calculated using the following equation:

F_F = μ x W

where F_F is the frictional force, μ is the coefficient of friction, and W is the weight of the object. When an object is not sliding, μ_s is used, which stands for coefficient of static friction. Conversely, μ_k is used for sliding objects and stands for the coefficient of kinetic friction. Note that μs will always be larger than μ_k. The values for μ are usually found experimentally. In this activity, a method of measuring μ_s will be introduced.

Before the Activity

Diagram of the activity setup. The box's friction with the table keeps the basket from falling to the floor.
click for copyright

Collect materials.
Print out copies of the Sliders Worksheet.
Designate areas and tables for the students to work.

With the Students

To get students to think about the activity, brainstorm with them. Ask the students give examples of good and bad friction. See Assessment section for examples.
Gather the materials needed.
Record the weight of the box (in grams) in the worksheet table.
Tie the fishing line around the sides of the box (see diagram).

Position the fishing line on the lower half of the box to prevent the box from tipping over.
Once in place, tape the line in place so it does not fall off or shift during testing.

Tie the other end of the line to a basket and hang the basket off the edge of a table (see diagram).
Place 500 grams of weight in the box and gently add weight in the basket, which should be hanging from the line off the side of the table.

Caution: If the weight is added ttoo quickly or too roughly, it may cause the box to start sliding, and ruin your data.

Keep gently adding weight until the box starts to slide.

To increase accuracy, add small amounts of weight (~10g-20 g) at a time.

Weigh the basket and record the weight on the Sliders Worksheet.
Ask the groups why they think fishing line is being used instead of rope or string. Explain that this is because the fishing line is very thin and smooth, which makes any friction between the table and the line extremely small. So small that it can be ruled out. If we used rope or string, we might have to take into account its frictional effect.
Repeat for 1000 grams, 1500 grams, and 2000 grams of weight in the box.
Calculate the coefficient of static friction for each trial.

μ_s = weightbasket ÷ weightbox
In this experiment, F_F equals the weight of the basket.

Average the four values.
You now have the coefficient of static friction between the table and the box.
Conclude by conducting the "Pass the Buck" activity from the Assessment post-activity section and asking the discussion question.

Sliders Worksheet

Pre-Activity Assessment

Brainstorming: In small groups, have the students engage in open discussion. Remind students that no idea or suggestion is "silly." All ideas should be respectfully heard. Ask the students to give examples of good and bad friction. (Answers: Good friction = between your sneakers and the ground, or your car tires and the road. Bad friction = in engines, causing them to heat up and break, or in your skateboard or rollerblade bearings, making it difficult to skate.

Activity Embedded Assessment

Group Question: During the activity, ask the groups:

Why are we using fishing line instead of rope or string? (Answer: The fishing line is very thin and smooth, resulting in any friction between the table and the line being extremely small — so small that it can be ruled out. If we used rope or string, we might have to take into account its frictional effects.)

Post-Activity Assessment

Pass the Buck: Now that the students have measured the coefficient of static friction for a box, have them brainstorm ideas to design an experimental test setup for testing the coefficient for a motorcycle's tires. First, assign one student in the group to be the recorder. Then have someone toss out an idea. Next, another person in the group provides an idea that builds on the first. Go around the group in this fashion until all students have put in enough ideas to put together a design. When they are done, have them share their ideas with the class. (Example answer: Tie a rope to the motorcycle and see how many people it takes to move the motorcycle. You could test again with a person sitting on the motorcycle and then average the coefficients of friction.)

Discussion Question: Solicit, integrate and summarize student responses.

What would happen if the kinetic friction of the box was stronger than the static friction? (Answer: If the kinetic friction was stronger, once the box started sliding its friction would increase and stop the box from sliding. Once the box would come to a stop, its friction would decrease again causing an infinite loop of starting and stopping. Now wouldn't that be fun to watch?)

Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder Chris Yakacki, Malinda Schaefer Zarske, Denise Carlson © 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last Modified: April 27, 2006