Lab Exercise 3: Friction

Follow the instructions and directions below for this lab.  Disregard the outline in the manual for your LabPaq Kit.

Read this document entirely before starting your work.

Do not forget to record your measurements and partial results.

Submit a Laboratory Report through Moodle, as shown in the last section of this outline.  Remember that the Laboratory Report should include the answers to the questions below.

GOALS

(1) To examine the concept of friction.

(2) To calculate the coefficient of friction of an object.

INTRODUCTION

Friction is an important force, with positive and negative implications. Whenever two objects slide along each other, friction is involved; whether a skier is sliding down an icy slope, or a crate is dragged across the floor. On the positive side, friction between the road surface and the car tires is what keeps the car moving along a highway. On the negative side friction reduces the efficiency of machines. As more work needs to be done to overcome friction, this extra work is wasted as it is dissipated in the form of heat energy.

When one surface slides over another, a resisting force, friction, is encountered. Friction, and the force needed to overcome it depends on the nature of the materials in contact with each other and on the roughness or smoothness of the contact surfaces. It is also affected by the normal force (FN), but not by the contact area or on the speed of the motion. It has been determined experimentally that the force of friction (Ffr) is directly proportional to the normal force (FN). When an object is resting on a horizontal surface the normal force is just the weight of the object (mg). If an object is on an incline, FN has to be corrected via the equation FN= mg cos θ with θ being the angle of the incline.  In this experiment, we will only measure friction on a horizontal surface, so we don’t need to concern ourselves with the issues affecting friction on an incline.

The constant of proportionality is called the coefficient of friction, μ. The force of friction when two contacting surfaces are sliding over each other can be calculated by:

where Ffr  is the force of friction;  FN is the normal force; and μk or μs are the coefficient of friction, which is a proportionality constant. The force of friction is parallel to the contact surfaces and opposite to the direction of motion. The term μk stands for coefficient of kinetic (or sliding) friction, which applies when the surfaces are moving with respect to each other.

When an object is at rest on a surface and we attempt to push it, the frictional force is opposing the pushing force. As long as the pushing force is less than the friction force, the object will not move. There is a threshold value of the pushing force beyond which a larger pushing force will cause the body to start sliding. It is this threshold value which is related to the coefficient of static friction, μs.   However, when an object is already in movement, the friction that it experiences is related to its coefficient of kinetic friction, μk.   When comparing published μk and μs values for identical materials, we see that μs is slightly larger than μk .  This indicates that it takes more force to start moving a material than keeping it moving.

PROCEDURE

For purposes of data recording, we will use Newtons (N) as this is the unit of Force in the International Systems of Units. For the purposes of this experiment, we can convert any mass to force using the following formula:

Measurement setup

For each measurement setup described in this section, we will measure two forces.  First, we will measure the force that we need to exert on a wooden block so it starts moving from a resting position.  This force is related to the coefficient of static friction.  The second force is the force that we need to exert on the wooden block to keep it moving. This force is related to the coefficient of dynamic friction.

Because the wooden blocks in your lab kit are too light to produce accurate results, we will augment their mass by adding an object of constant weight, for example a can of soda. The experimental setup can be seen in figure 1 below.  You can use any other object that you may have around instead of the soda can.

       Figure 1:  Experimental  Setup

We will perform two types of measurements:

Force of Static Friction  (Fs):  Starting from the block at rest, pull from the spring scale until the block starts moving. Record this force.

Force of Kinetic Friction (Fk):  Starting from the block at rest, pull from the spring scale until the block starts moving and continue pulling until it moves at a constant speed. Record this force.

You should observe that Fk is lower than Fs.We will repeat these measurements using several materials as well as in different configurations for the wooden blocks.

Question 1

What is the mass of the system made of the wooden block and the soda can (or the other object that you are using)?

Question 2

Convert the mass measured in Question 1 to its weight in Newtons.  This is the value of FN that will be used in the calculations for the tables.

SURFACES

Wood / Wood (larger surface)

For this case, we will use the larger surface of the wooden block as was shown in Figure 1.  Measure Fs and Fk as indicated in Section 3.1.   Run 5 trials, completing Table 1 below.

Wood / Wood (smaller surface)

For this section, turn the wooden block on its side as shown in Figure 2 and repeat the experiment, completing Table 2.

Figure 2:  Wooden block on its side  (courtesy of Chad Saunders, TESU student)

Wood / Glass

For this section, use the glass surface of your wooden block to repeat the measurements and complete Table 3.

Other surfaces

Repeat the measurements using other surfaces (for example, Wood / Sandpaper, Wood / Carpet, Glass / Carpet, etc.)  When completing Table 4 below, make sure that you indicate the surfaces you used.

ANALYSIS OF RESULTS

QUESTION 3

Study the results from Table 1 and Table 2.  What can you conclude about these results?

QUESTION 4

Using the data from the same tables, do you think that the ratio of μk to μs is constant in both cases?   If so, what do you think this indicates?

QUESTION 5

Studying the standard deviation data from all the 4 tables, which experiment do you think is the most reproducible?  Why?

QUESTION 6  

In general, how does the coefficient of static friction compare to the coefficient of dynamic friction?

QUESTION 7  

In designing machinery, would we prefer to use materials with larger or smaller coefficient of friction?  Explain your reasoning.

QUESTION 8  

In driving a vehicle, would you prefer to use materials for the contact between the wheels and the road with larger or smaller coefficient of friction?  Explain your reasoning.

LABORATORY REPORT

Create a laboratory report  using Word or another word processing software  that contains at least these elements:

Introduction:  what is the purpose of this laboratory experiment?

Description of how you performed the different parts of this exercise.  At the very least, this part should contain the answers to questions 1-8 above.  You should also include procedures, etc.   Adding pictures to your lab report showing your work as needed always increases the value of the report.

Conclusion: What area(s) you had difficulties with in the lab; what you learned in this experiment; how it applies to your coursework and any other comments.

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