Name:________________________________________ Measuring the Coefficient of Friction Objective The purpose of this lab is to experimentally determine and compare the coefficient of static friction and the coefficient of kinetic friction; compare friction vs. area; and compare friction of different materials. Background The coefficient of friction is a ratio that is calculated by dividing the frictional force by the normal force. If an object is at rest, or pulled at a constant speed across a level surface the frictional fore will equal the applied force and the normal force will equal the weight of the object. Before the object begins to slide the friction holding it in place is called “static friction.” Once an object is moving, it requires less force to maintain a constant speed. The frictional force of a moving object is called “kinetic friction.” Kinetic friction is usually less than the maximum static friction because the frictional force is smaller when the surfaces are sliding past each other. Materials LabQuest, Dual-Range Force Sensor, String, Friction Box, Wood Board, Masses. Procedure: 1. Mass your box and record the value on your data table. 2.

Make sure Force sensor is set to the 10 N setting and is plugged into channel 1 on the LabQuest.

3.

Hold the force probe vertically and hood a 500 g mass on the hook. The probe should read 4.9 N. If not, you will need to calibrate the probe. Get Help!!!

4.

Attach the force probe to the friction box with a loop of physics string. Place the box on the wood board with the “bottom” down.

5.

Add a 100 g mass to the box. Record the mass on your data table.

6.

With no force pulling on the sensor, and holding it horizontally, “ZERO” the probe.

7.

Click the “Collect” button and slowly start to pull horizontally on the object using the sensor. Once it begins to move try to pull it at constant speed. If the mass moves in the box, redo the trial. It is VERY important the string is horizontal with the board as you pull the box.

8.

Re-scale the graph if you need to. It should resemble the graph shown below.

9.

Click and drag to select a small region that has the max static friction within it. Under the Analyze menu, select STAT and Force. Record the maximum value from this region as force of static friction on your data table.

10. Click and drag to select the region that is roughly horizontal which represents the kinetic friction. Under the Analyze menu, select STAT and Force. Record the mean value for this region as the force of kinetic friction on your data table. 11. Repeat steps 7 – 10 by adding masses which cause the force probe to reads a significantly different value. You must be careful however to never exceed the 10-N max reading of the sensor. Each group member is responsible for analyzing the data and answering the questions. You can get help, but you will be held accountable for knowing how to do this on quizzes and the unit. I. Kinetic vs. Static Friction Mass of Wood Box (g) Total Mass (g)

Total Mass (kg)

Normal Force (N)

Force of Static Friction (N)

mean

μs

Force of Kinetic Friction (N)

μk

mean

Questions Each individual lab member is required to answer the following questions in complete sentences. Your responses should written neatly. 1. Draw a free body diagram for the forces acting on the box.

2. How did the size of the frictional forces change as more weight was added to the box?

3. For each trial, calculate μs and μk. Record the values in the table. Calculate the average.

4. How does the value for the coefficient of friction change as you add more weight to the box?

5. How does the magnitude of static friction compare to the magnitude of kinetic friction?

6. Why is it important to pull the object at a constant speed? What happens if you don’t?

7. Why is the equation for static friction defined as fs ≤ μsN (rather than fs = μsN)?

8. How would the use of a motion detector be beneficial in this lab?

9. Use a graphing calculator to graph Friction vs. Normal force for both sets of data. Use linear regression to find the slope of the lines. Compare the slope values with the average values calculate for the coefficients of static and kinetic friction.

II. Comparing Friction and Area In this lab, you will set the box on its “side” and repeat the procedure from lab I. Pre-lab. When the box is set on its “side” there is less area making contact to the board. Will this make the force of friction increase, decrease, or remain the same as the force when it is setting on its “bottom”? Write a few sentence to explain your thoughts.

Procedure. Repeat the same data collection steps as lab I. This time however, we are only interested in the static friction values. II. Comparing Friction and Area Mass of Wood Box (g) Total Mass (g)

Total Mass (kg)

Normal Force (N)

Side Static Friction (N)

Questions Each individual lab member is required to answer the following questions in complete sentences. Your responses should written neatly. 10. Use a graphing calculator to graph Friction vs. Normal force for the data. Use linear regression to find the slope of the line. Compare the slope value with the slope for the static friction graph in lab 1 and your pre-lab conjecture. Is there a significant change in the value of the coefficient?

III. Comparing Friction and Surface. In this lab, you will tape sand paper to the bottom of your box and repeat the data collection and analysis. Procedure. Repeat the same data collection steps as lab I. This time however, we are only interested in the static friction values. III. Comparing Friction and Area Mass of Wood Box (g) Total Mass (g)

Total Mass (kg)

Normal Force (N)

Static Friction (N)

Questions Each individual lab member is required to answer the following questions in complete sentences. Your responses should written neatly. 11. Use a graphing calculator to graph Friction vs. Normal force for the data. Use linear regression to find the slope of the line. Compare the slope value with the slope for the static friction graph in lab. Is there a significant change in the value of the coefficient?