Coefficients of Friction

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AP Physics LaboratoryCoefficients of Friction


Friction is the force that resists motion and can be determined in either static situations or kinetic situations using the following equation:


Static friction is the amount of friction that resists the start of motion and kinetic friction resists an object’s continued motion. The coefficient of friction depends on a number of factors including surface area and the types of surfaces in contact. The only way to determine a coefficient of friction is experimentally. (Note: if a graph of Ff vs. N is draw, the slope of the graph is μ).

The angle of repose is defined as the angle at which an object just starts to slide down an inclined plane. If θ is the angle of repose, it can be shown that μ = tan θ. We will call this method to determine the static coefficient of friction the angle of repose method.

In this experiment you will determine the coefficient of kinetic and static friction between the rubber stoppers on your calculator and the aluminum dynamics cart track.

The Experiment:

1. Find the coefficient of static friction by slowly raising the incline plane until the block just begins to slide. Note the angle. Repeat 3 times and average to accurately find the angle of repose.

2. Determine a method in which the coefficient of kinetic friction can be determined.

3. Perform your experiment to determine the coefficient of kinetic friction for the rubber stoppers on your calculator cover on the aluminum track.


In addition to any necessary measurements or calculations from the experiment itself, include the following in your lab report.

4. Outline your methodology for finding the coefficient of kinetic friction between the rubber feet on a calculator and the aluminum track. 5. Apply Newton’s 2nd Law and show that μs = tan θ on incline planes. 6. Explain what happens to the angle of repose if the mass on the incline plane is doubled.
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