Friction Lab Report

I.  Introduction

For this experiment, we calculated the force of both static and kinetic friction on horizontal and inclined surfaces. We tested two surfaces of different textures; rubber and Teflon. An aluminum block was placed on top of the surface, and a motor pulled the block across the surfaces for the horizontal test. The force was measured by an electronic sensor, and data was then graphed in the computer. For the tests on inclined surfaces we used a pulley to increase the angle the board made with the ground until this block slid down the incline. At the point the block began to slide was the angle which correlated to the coefficient of static friction.

                        fs  = us n or fk = uk n   and us = tan (θ)

II. Data Analysis

II.1 Measurement of the Force of Friction (Horizontal)

In this experiment we measured the force required to make to aluminum block begin sliding over both a Teflon and rubber surface. This test was run with three different masses added onto the aluminum block, to demonstrate the effects of normal force on the force of friction. Data was collected by the electronic sensor, and was displayed graphically on the computer. Three runs of this test were completed for each different mass, for both surfaces. The computer allowed us to measure both the static friction and kinetic friction. Our data supports that the coefficient of kinetic friction is a lower value than the coefficient for static friction.

II.2 Measurement of the Force of Friction (Inclined)

To measure the force of friction acting on the aluminum block on an incline plane, we measured the angle at which the block began to slide. Using a system with a pulley and a string, the board, with the Teflon surface attached, was raised smoothly at a constant speed. The instant the board began to slide was the angle which corresponds to the coefficient of static friction. In order to calculate the coefficient of static friction, we took the tangent of the angle:

Sample Calculation

Angle θ (degrees): 13o

us = tan (θ) = tan (13o) = 0.23

Because the coefficient of friction does not change when additional mass is added, we averaged all values and calculated the average coefficient of static friction of  aluminum and Teflon to be 0.22.

II.3 Determining the Coefficient of Friction

When testing for the coefficient of friction on the non-horizontal Teflon surface we were able to use the angle to calculate the coefficient. However, when we were testing on the horizontal surfaces, our measurements were of the force required to get the aluminum block to move, and to keep it moving. To calculate the coefficients of static and kinetic friction we had to plot our data, and calculate the slope the line of best fit. The slope of the two lines were the coefficients of friction.

Sample Calculation

For the graph of static friction as a function of normal force,

Coefficient of Static Friction = Slope = (y2 – y1)/(x2 – x1)

Points:  (0, 0) and (19, 3)

Which yields:

(y2 – y1)/(x2 – x1) = (3 – 0)/(19 – 0) = 3/19 = 0.16

Therefore, from our data, the coefficient of static friction for aluminum and Teflon is 0.16.

III. Conclusion

Through the use of both a horizontal and non-horizontal plane, we were able to calculate the forces and coefficients of both static and kinetic friction. While our data is not entirely linear, the line of best fit displays the relationship between normal force and the force of friction. The slope of the line of best fit is the coefficient of friction. My graph allowed me to measure the coefficient of static friction between aluminum and Teflon to be 0.16, and the coefficient of kinetic friction to be 0.12. Our data also helped us to prove that the coefficient of friction, both static and kinetic, is independent of mass. While the force of friction will increase when the normal force increases, the coefficient of friction will remain constant.