What Variables Determine the Amount of Friction?

Grade Level/Subject / 11th grade physics
Unit / Force
Enduring Understanding / The force of friction that exists between two objects is determined by the types of materials and the normal force that is exerted between them.
SOL Objectives / PH.3 The student will investigate and understand how to demonstrate scientific reasoning and logic. Key concepts include
e)  construction and defense of a scientific viewpoint (the nature of science).
PH.5 The student will investigate and understand the interrelationships among mass, distance, force, and time through mathematical and experimental processes. Key concepts include
d)  Newton’s laws of motion;
Title / Phun with Phriction – A Miniseries of Lab Activities
Lesson Objective / Students will investigate several variables that may influence the amount of friction that exists between two objects.
Inquiry Level / 2 – The question and methods are provided but the solution remains open.
Materials Required / ·  Spring scales (5-20 Newton)
·  Old textbooks (blocks of wood may be used instead)
·  Friction boards with multiple surfaces
·  (4) 500g masses
·  Small wood blocks with a screw-eye attached at one end
·  protractor

Teacher Supplemental

·  It may be helpful to let the students suggest ideas about what variables may affect the amount of friction that is present when two objects are in contact. They will usually suggest “type of material”.

·  Students will often suggest that velocity affects the amount of friction. This is a good opportunity for them to interpret a null result. Many students will see the result that they expect to see. Make sure that they do multiple trials.

·  It is very difficult to read the spring scale accurately while moving quickly. Suggest that the students make their “slow” speed very slow, so that the medium and fast speeds are reasonable.

·  Tying a loop of fishing line and slipping it halfway through the textbook (so that the string pulls on the binding) is a quick and easy way to attach a string to a book. You can then hook a spring scale onto the string.

·  Students generally need support in seeing that the pulling angle determines normal force, and that it is normal force that determines the amount of friction. A follow-up discussion is helpful.

Phun with Phriction – A Miniseries of Lab Activities

Our goal today is to learn about the nature of friction through a series of small experiments. We will be focusing on objects with constant velocity, allowing us to make use of direct measurement with spring scales.

In actuality we cannot directly measure the force of friction with a spring scale, but because of Newton’s 2nd and 3rd laws, we can easily determine it. In the space below, complete the force diagram to show how the force measured on the scale is equivalent to the force of friction (Assuming that the book is moving at constant velocity).

1.  Surface Area: We know that friction occurs between two surfaces. Does the amount of contact surface area affect the amount of friction force? How can we answer this, using the book and spring scale?

2.  Describe your findings.

3.  Type of surface Use the special friction boards and measure the friction between the wooden “sled” and the four different types of materials. (you may need to place a 200g mass on the sled to get good results).

material / Force of friction
cardboard
cork
rubber
sandpaper

4.  Examine the different materials. Describe characteristics of these materials that may affect the amount of friction.

5.  Mass: Does the object’s mass affect the amount of friction force? If so, what is the relationship?

Repeat the previous experiment. Then measure it again, adding mass in 500g increments... as many as you can reasonably fit.

Mass of object / Ffr with 500g / Ffr with 1000g / Ffr with 1500g / Ffr with 2000g
cardboard
cork
rubber
sandpaper

6.  What does this suggest to you about the relationship between mass and friction force?

7.  Different angles of pulling force: Measure the force of friction between a book and the floor with the following scenarios. Pull with a 10 or 20 N spring scale.

Pulling angle / FP (from spring scale) / Friction (FPcosθ)
0°
10°
20°
30°
40°
50°
60°
70°

8.  Complete the following free-body force diagram. Label all of the force vectors (be careful with the angle!) NOTE: This is a generic diagram, suitable for any angle θ.

9.  Look carefully at your diagram. The pulling angle indirectly influences the amount of friction. What variable is actually determining the amount of friction?

10.  Velocity: Is the force of friction related to the object’s velocity?

Measure the force of friction as the book moves at different constant velocities.

Velocity / Force of Friction
slow
medium
fast

11.  Describe this relationship.

12.  Summary: Summarize your conclusions about each of the different factors that you tested.

·  Surface area

·  Type of surface

·  Mass

·  Pulling at an angle

·  Velocity

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