AP Physics Laboratory
Coefficients of Friction
DISCUSSION:
Friction is the force that resists motion and can be determined in either static situations or kinetic
situations using the following equation:
F f  N
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.
ANALYSIS:
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.