Teacher Guide

# Friction Fun

## Experimenting with different surfaces and friction

What affects the force of friction?

This resource was originally published in PhysicsQuest 2018: Force.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Friction Fun

What affects the force of friction?

• Rough grain sandpaper
• Fine grain sandpaper
• Four nuts
• Fun dough
• Rubber bands
• Ruler
• Tape

Students start by discussing a problem that prompts them to consider the effects of friction. Then, they experiment, collect, and analyze data. They use this analysis to discuss and predict which surfaces create the most friction.

• Total time
45 - 60 Minutes
• Education level
Grades 5 - 9
• Content Area
Force
• Educational topic
Friction, kinetic energy

It’s friction that holds you against the side and stops you from falling down with the floor. As two surfaces move against each other, they lose some energy to friction.

Friction is a force that resists movement. The classic example is rubbing your hands together and feeling them get hot. Friction is turning some of the energy from rubbing your hands into the heat energy you feel. Any time two surfaces are moving when smooshed against each other, there will be some friction. For any combination of surface and object there is a maximum amount of force that friction can apply. To get an object to move on that surface, the force you are applying has to be greater than the maximum amount of force friction can exert. The maximum amount of frictional force depends on how heavy the mass is and how rough the surface is. Think about a 100-pound box sliding on carpet versus a 10-pound box on ice. You have to push harder on the heavy box on carpet because the maximum frictional force in that system is high. It wouldn’t take near as much to overcome the maximum friction of a box on ice. (Fig. 1)

It’s possible to measure this maximum frictional force for a specific mass and surface by figuring out how much force is required to just barely start the object moving. That’s the point at which the maximum frictional force is overcome and the object starts sliding at a constant rate. If you can find a way to figure out how much force is applied at that point, then you also know how much force friction can exert. By comparing how that max force changes with surfaces and weights, you get a good idea of how those two things change frictional force.

For any surface there is a number, call the coefficient of friction, that says how much frictional force is exerted for any given weight. If you know this number and you know the weight of something, you can find the maximum force of friction. The rougher the surface, the higher this number will be.

In this activity, students will measure how much force it takes to move different weights — in this case, nuts — two different surfaces. It’s hard to find something that will fit in a PhysicsQuest kit that can directly measure force. Instead, the students will be using rubber bands and looking at how far they stretch before the mass of nuts moves. I don’t want to go too far on a tangent about how rubber bands work and the potential energy involved, so I’ll sum up how they are used to measure relative forces.

When a rubber band is stretched, it has potential energy. That stretch also exerts a force on whatever it is attached to. The more the rubber band is stretched, the more energy it has and the more force it is exerting on anything it is attached to. If the object isn’t moving, it means it is exerting the same force as the rubber band — only in the opposite direction.

In the case of rubber bands and nuts, if the rubber band is being stretched and the nuts aren’t moving it means that friction is exerting the same amount of force as the rubber band in the opposite direction. When the rubber band is stretched to the point that the buts begin to move, it is applying exactly the same amount of force as the maximum amount of frictional force.

By measuring the length of the rubber band right at the moment of movement for different systems, you can see how the maximum amount of frictional force changes from one system to the other. This certainly isn’t the most precise way to do it, but it is a great way to introduce students to the different things that affect friction.

Key terms

These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.

• Friction: The resistance to motion of one object moving against another. Rougher surfaces moving against each other have more friction.
• Force: Something that changes the motion of an object. Pushing or pulling is a force; so is friction.
• Coefficient of friction: A number that tells how strong frictional force will be for a particular weight. Different surfaces have different coefficients of friction. Ice has a lower coefficient of friction than sandpaper.
Objective

Students will experiment and calculate friction.

Before the experiment
• What would be harder: pushing a box across a shiny wood floor or pushing a box across carpet? Why?

1. Pair students up.
2. Give them a minute to think quietly.
3. Give students 2 minutes to discuss their thinking.
4. Have students record their answers or share out to the whole group.

Setting up
• Take the rubber bands and hook the loop one through the other to attach them together.

• Take one end of the rubber bands and loop it through one nut to attach the rubber bands to the nut.

• Surround the nut with fun dough, making sure the rubber band is sticking out.

• Tape the two strips of sandpaper next to each other on a table or desk.

• Set down a ruler along the sandpaper strips.

During the experiment
Collecting data
• Make sure students are put into intentional groups. See above.

• Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.

Analyzing data
• In the student guide, they will answer questions that help them understand friction.

• Continue to listen in on each group’s discussion; answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Teacher tip

Suggested STEP UP Everyday Actions to incorporate into activity:

• When pairing students, try to have male/female partners and invite female students to share their ideas first.
• As you put students into groups, consider having female or minority students take the leadership role.
• Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
• Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
• As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.

Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.

Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives.

Conclusion
• Post the conclusion questions.

1. Do you think ice has a high or low coefficient of friction?
2. Do you think cardboard has a high or low coefficient of friction?
3. Do you think asphalt has a high or low coefficient of friction?
4. Do you think wood floors have a high or low coefficient of friction?
• Use the Gallery walk protocol to have students share and refine their thinking.

1. Posters with each question or surface option are set up around the classroom, on the walls, or on tables.
2. Small groups of students travel from station to station together, discussing each question, and writing their answers on individual paper or group paper.
• After students have had a chance to discuss key ideas from the lesson and complete their student guides, you can now clarify and give concise definitions to the forces they experimented with.

• Real world connections:
• Using what you learned about coefficients of friction, how does a zamboni help ice skaters, or a hockey puck move across the ice?
• Suggestions for drawing, illustrating, presenting content in creative ways:
• Engineering and design challenges connected to the content:
• In this experiment, students should learn about different objects that have different coefficients of friction. Using toy cars/marbles and a racetrack/flat surface, have students add various surfaces to a racetrack to try and win the race. Examples include sandpaper, bubble solution, glue, oil, etc.
• MS-PS2-2
Plan an investigation to provide evidence that the change in an object’s motion depends of the sum of the forces on the object and the mass of the object.
• MS-PS4-3-applications
CCC: Influence of Science, Engineering, and Technology on Society and the Natural World. Technologies extend the measurement, exploration, modeling, and computational capacity of scientific investigations. (MS-PS4-3)
• MS-PS4-3-nature-of-science
CCC: Science is a Human Endeavor. Advances in technology influence the progress of science and science has influenced advances in technology. (MS-PS4-3)
• MS-PS4-1-empirical-evidence
SEPs: Scientific Knowledge is Based on Empirical Evidence. Science knowledge is based upon logical and conceptual connections between evidence and explanations. (MS-PS4-1)

### Credits

Created by Rebecca Thompson, PhD, Scott Arnold, Roel Torres

Updated in 2023 by Sierra Crandell, MEd, partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)

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