Friction Is Not Fiction!

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Students explore what factors affect the amount of friction between two objects and use the friction equation to solve a variety of problems.

Big Idea

The force of friction between two objects is determined by the normal force and the properties of the surfaces in contact.

Context and Supplies

Friction is the next topic in our study of Newton's Laws of motion. In a previous lesson, students used F=ma and calculate the weight of an object; now it is time for them to learn about the friction equation. This lesson starts with students exploring what factors affect the force of friction. Then a short lecture gives them the friction equation where they learn about the coefficient of friction. Finally, students use the friction equation to determine the coefficient between a block of wood and their lab benches. 

Because of the mathematics involved in the problems and activity, Math Practice 2: Reason abstractly and quantitatively is a CCSS for this lesson along with NGSS  Science Practice 5: Using mathematics and computational thinking. Students will work in groups to plan and carry out an investigation which is Science Practice 3. This is all in the context of NGSS Performance Standard HS-PS2-1, the mathematical relationship between the net force acting on an object and its acceleration.

The following supplies are used in this lesson:

  • 12 wood blocks with an eye hook at one end
  • 12 spring scales (range of 0-5N)
  • 12 500-g Masses

The supplies are at the front of the room for the groups to collect and then return at the end of the period. The spring scale is used to drag the block so that one can read the force required to pull the wood block at a constant velocity. Since the velocity is constant, the spring scale measures the force of friction between the wood block and the table. The 500 gram masses are placed on top of the wood block to increase the friction force; this puts the friction value near the middle of the range of values on the spring scale. 

Students Friction Exploration

15 minutes

As students enter the room, on the board are the day's objectives:

  • Explain the factors that influence the force of friction
  • Calculate the coefficient of friction between two surfaces

I tell students that they will be using the supplies to determine the force of friction between two surfaces. But first, they work independently to complete the Friction Exploration sheet. This sheet has students think logically about the factors involved with friction between two surfaces, primarily by comparing two objects, a large box of books and a large box of feathers. This is a compare and contrast teaching strategy which is a highly effective tool for student learning. I have them work independently because most students can complete this exercise without help. 

When students finish number 8 on the sheet, they bring it to me and I check it for correct answers using the friction exploration solution sheet. If there are mistakes, I point them out and send them back to their desks to make corrections. If students have correct answers then they move ahead to number 11 on the sheet (one does not need to understand coefficient of friction to look up values). Questions 9 and 10 will be completed after the lecture. 

Lecture on Friction Sesame Street Style

15 minutes

Once the majority of the class has finished the first 8 questions of the worksheet, I begin the lecture. I frequently use humor as this keeps the students entertained and this increases intrinsic motivation to be on task. This lecture brings smiles to students faces as I incorporate a Sesame Street themes into it. I blast the Sesame Street theme song at the beginning of the Friction Lecture Power Point and I say that "Today's class is brought to you by the letter mu and number one". The power point has animations that are best viewed if it is downloaded. During this lecture, students are taking notes and working the sample problems given in the Power Point. The presentation introduces them to the the friction equation, the coefficient of friction, sample problems and free body diagrams.

When the lecture is done, I instruct the students to finish the Friction Exploration worksheet that was started earlier in the class and to turn it in as soon as they finish. The problems at the end of the sheet apply the concepts students learned during the lecture. I think it is important to give students an opportunity to practice new concepts as soon as they are learned.

Students Determine Coefficient of Friction

20 minutes

This activity has the students using what was presented in the lecture to determine the coefficient of friction between the block of wood and the tables. As students bring up their completed Friction Exploration worksheets, I check them for correct answers. If everything is correct, then I give the student the sheet back and they can move onto the next activity. Students cannot move on until they succesfully complete the worksheet.

Once 4 students successfully complete the worksheet, I group them together to work together on the Friction Activity Handout. I give them the block of wood and the spring scale (this video shows students dragging a wood block). Each student has a specific role to perform so that everyone is involved in the completion of the assignment. Research shows that cooperative learning yields a positive effect on overall learning, especially when roles are assigned in the completion of the activity. 

Because the best students usually finish first, they are grouped together. I give direct support to the struggling students, so in this activity (solution to challenge problem), homogeneous grouping is best. This gives the more advanced groups a chance to attempt the advanced problem on the back of the activity.

I collect this activity for grading at the end of the period. If there is a group that does not finish, they come in to complete it during a free period. In addition, I give the Friction Homework sheet to the students as they pass in their activities sheets. This provides more practice problems and also has students think about the difference between static and kinetic friction.