SummaryIn 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 and the coefficient of kinetic 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. Clearly, engineers really need to know their friction facts!
After this activity, students should be able to:
- Understand how static and kinetic friction are important concepts in engineering design
- Collect data to solve equations.
- Experimentally measure the coefficient of static friction and kinetic friction.
- Recognize that different surfaces have different frictional coefficients.
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Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.
All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).
In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.
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Each group needs:
- 3 ft. fishing line
- Small, stiff cardboard box approximately 5 in. x 5 in. x 5 in. or a Tupperware® container of similar size
- Stop watch, or watch with second hand
- Weights (see Troubleshooting Tips)
- Small wicker or plastic basket with a handle
- Desk or table
- 2 copies of the Slip Slidin' Away Worksheet
For class to share:
- Weight scale
- Tape (any kind)
Recall Newton's 2nd Law: the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. One of the considerations of net force is the frictional force that can either inhibit the initial motion of a still object or cause a moving object to slow down to a resting position.
Friction can waste energy, wear down parts and cause things to heat up. But, it is not always a bad thing! In fact, friction can be a reliable ally. For example, we depend on friction to keep our shoes/feet from sliding out from under us (and causing us to fall) and to keep our cars on the road (from friction under our car's tires). When friction is removed from these situations, such as when there is ice on the road or sidewalks, disastrous results 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 did not exist, it would feel 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 sliding freely 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:
FF = μ x W
where FF is the frictional force, μ is the coefficient of friction, and W is the weight of the object. When an object is not sliding, μs is used, which stands for coefficient of static friction. Conversely, μk is used for sliding objects and stands for the coefficient of kinetic friction. Note that μs will always be larger than μk. The values for μ are usually found experimentally. In this activity, a method of measuring μs will be introduced.
To calculate the coefficient of kinetic friction, we need to create a model to calculate the change in energy. Remember, energy is often measured as: Work = Force x Distance. If we put a certain amount of energy into the system, and then stop, the distance the box takes to stop times the frictional forces on the box can tell us the work done by friction.
Before the Activity
- Collect materials.
- Print out copies of the Slip Slidin' Away Worksheet.
- Designate areas and tables for the students to work.
With the Students
- To get students thinking about the activity, brainstorm with them. Ask the students to give examples of good and bad friction. See Assessment section for examples.
- Pass out materials to student groups.
- Ask students to record the weight of the box and the basket (in kg) on their Slip Slidin' Away Worksheet.
- Instruct students to tie the fishing line around the sides of the box (see Figure 1).
- 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 that it does not slip off or shift during testing.
- Have students tie the other end of the line to the basket handle and hang the basket off the edge of a table, as illustrated in Figure 1. Place the box in the center of a desk/table.
- Students should place 500 grams of weight in the box, and gently add weight to the basket (which should be hanging from the fishing line off the side of the table). Note: If the weight is added too quickly or too roughly, it may cause the box to start sliding and ruin your data.
- Ask students to continue to gently and gradually add weight until the box starts to slide. To increase accuracy, they should add small amounts of weight (~1g-10 g, depending on available resources) at a time.
- Instruct students to record the total amount of weight in the basket that finally caused the box to begin sliding.
- Ask the groups why they think fishing line is being used instead of rope or string. (Answer: 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 would have to take into account its frictional effect.)
- Repeat steps 6-8 with 1000 grams, 1500 grams and 2000 grams of weight in the box.
- Ask students to calculate the coefficient of static friction for each trial.
- Students should average the four values. They now have the coefficient of static friction between the table and the box.
- Set up the experiment again, but ask students to ensure the fishing line is long enough so that the basket can touch the floor while the box remains in the middle of the desk/table.
- Measure the distance from the edge of the box to the table edge and then the distance from the floor to the bottom of the basket. Note: Have students use a piece of tape or washable marker to mark the starting position of the box so that they do not have to re-measure it for every trial.
- Place 500 grams of weight in the box.
- Have one group member hold the basket in place, while another group member gently adds more weight (~50 grams – 200 grams) to the basket than the amount that caused the box to slide (overcome static friction) in the previous trials.
- The student holding the basket should release it and the group should time how long it takes for the basket to hit the floor.
- Instruct students to measure the distance from the edge of the box to the table edge after basket has landed on the floor.
- Repeat step 15-18 with 1000 grams, 1500 grams and 2000 grams of weight in the box.
- Ask students to calculate the coefficient of kinetic friction for each weight in the box and find the average.
- Complete the Further Learning section on the Slip Slidin' Away Worksheet.
Make sure students are not wearing open-toed shoes, as weights could drop and injure their feet.
If weights are unavailable, common objects — such as coins, books canned food — can be used in their place, as long as they are each weighed. It is not necessary to have weights specifically ranging from 500g to 2000g exactly; however, the experiment will be more accurate if large weight values are used.
Use a box that weighs more than the basket.
For best results, try to replicate each trial exactly the same. Make sure the box is orientated in the same direction each time and that it starts the same distance from the table.
When calculating the coefficient of kinetic friction, students should place just enough weight in the basket so that the box will slide as the basket drops to the floor, but not so much that weights explode out of the basket from the impact of the floor.
For a more in depth example of the method being used to calculate the coefficient of kinetic friction refer to the Ramp and Review Activity from Lesson 5 of the Energy of Motion unit.
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 examples: between your sneakers and the ground, or your car tires and the road. Bad friction examples: 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.)
- How could we make the experiment even more accurate? (Answer: Consider all answers; answers will vary.)
Discussion: As a class, discuss Question 18 from the Sip Slidin' Away Worksheet. Also, ask students why they think engineers would design such an item? (Answer: For large public events, such as sports games, where organizers might want to quickly set up and take down an object designed for that event. This way they would also not have to worry about it being stolen or destroyed.)
Discussion Question: Solicit, integrate and summarize student responses.
Have the students graph their results.
- For lower grades, if students have not learned to use free-body diagrams, they may need help with Question 19.
- For older grades and if students have learned to use free-body diagrams, have them draw a free-body diagram of the box and basket for practice.
- For all grades, do activity as is.
ContributorsChris Yakacki; Malinda Schaefer Zarske; Denise Carlson; Ben Sprague; Janet Yowell
Copyright© 2006 by Regents of the University of Colorado.
Supporting ProgramIntegrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
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. 0338326. 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.