Friction Lab

Introduction

Friction is directly proportional to the normal force. Static friction refers to the amount of force required to get something to start moving. Kinetic friction refers to the force that opposes motion while something is moving. The purpose of this lab is to calculate the coefficients of static and kinetic friction between several surfaces. Because the coefficient of friction depends on both surfaces, changing either of the surfaces should change the coefficient. Also, you will determine how surface area and mass affect these coefficients.

• friction block
• several clean surfaces
• 0.5 kg masses
• newton (N) spring scales

Procedure

You must find the mass of your friction block (in kilograms!) before you begin. Use a spring scale to measure static friction as demonstrated by your teacher. Use a spring scale to measure the kinetic friction also as demonstrated by your teacher. Change the normal force by adding mass in increments of 0.5 kg. Measure the friction forces for at least four different surface combinations using four different masses for each combination of surfaces.

When measuring the friction force remember that performing each experiment three or four times will give you a better idea of the actual force than if you only did each one once.

The force of static friction is the amount of force shown on the spring scale at exactly the moment when the block just starts to move.

The force of kinetic friction is easier to measure. Use Newton’s First Law to justify your determinations.

Devise your own experiment to determine whether surface area of contact has an effect on the force of friction (either static or kinetic).

Analysis

1. Create a neat data table to contain all of the information you will need to perform the following analysis. Use the one on the back as an example. Record the data in your lab notebook in preparation for turning in a final draft quality version.
2. Calculate the coefficient of static friction (μ_s) and the coefficient of kinetic friction (μ_k) for each experimental setup.
3. Find the average μ_s and μ_k for a surface combination using the values found for each different mass/normal force.
4. Graph the static frictional force as a function of the normal force on one graph and the kinetic frictional force as a function of the normal force on another graph. Put all four surfaces onto each graph so that you have only two graphs total. Calculate the slopes for each surface and display them on the graph.
5. How do the slopes of your graphs compare with the average values of the coefficients? Can you explain this relationship?
6. How does mass affect the coefficients of friction? Justify your answer using experimental evidence.
7. How does surface area affect the coefficients of friction? Justify your answer using experimental evidence.
8. Is one coefficient of friction always larger than the other? If not, what is the trend: which coefficient tends to be larger? Explain this.
9. What are the units of coefficients of friction? Justify your answer mathematically.

To Hand In

Hand in your data tables, graphs, sample calculations and the results of the analysis requested above (that is, answer the questions). Do this by hand or on a computer but neatness counts. Show units and show all work for calculations and use a step-by-step approach.