Student No. xxxxx
May 19, 2009
This experiment measures the coefficient of static friction (μs) and kinetic friction (μk) between objects of different materials. Friction is a force that must be overcome before an object can move across a surface. A plain block of wood and a block of wood with sandpaper on one side and glass on the other were used. All of the blocks had a soup can with a mass of 0.41 kg placed on top in order to provide enough mass to allow readings to be taken. They were moved along a wood plank while being attached to a 500-g spring scale in order to record the values when a) the block first moved, representing μs and b) as it traveled at a constant speed, representing μk. In one experiment the wood block was placed on its side and the experiment repeated. Overall, the results showed that μs > μk, and that the block that had the least surface area on the plank also had lower coefficients of friction when compared to one with more surface area on the plank.
The purpose of this experiment is to observe the friction force and to determine the coefficient of kinetic friction as well as static friction of materials of different roughness. Various types of materials were used, as well as horizontal versus inclined ramps.
Friction occurs when two surfaces come into contact. The rough areas of each surface can come into contact and become cold-welded. Before an object can move over a surface, these cold-welds must be broken. It is a non-conservative force; the force used to overcome the frictional force and allow an object to move is dissipated into heat energy and will not return to the system once the movement stops.
Specifically, this lab will calculate the coefficient of friction. Unlike most coefficients in Physics, friction behaves differently depending on whether the object is at rest or at motion. (Nate). This is due to the cold-welds formed as discussed above. Once the object is in motion, cold-welds cannot form so therefore force is not needed to break them as in the case of a static object.
The first case, the static coefficient of friction, fs, is the force that keeps an object from moving. If there is an applied force to a block, and the block remains at rest, then fs = F. As the magnitude of F increases, fs will increase proportionally until is exceeded. It is proportional to the normal force, N, acting on the block. The basic equation for the coefficient of friction is: fs ≤ μsn
where μs is the coefficient of static friction. If the block is just about to move, then fs = fsmax = μsn. The force of friction is in the opposite direction of the movement. To calculate the coefficient of friction, the following equation will be used: μs = fsmax/FN
Friction is actually a much more complex force than it appears from this introduction, or from the following sets of experiments. Intuitively it would seem that the smoother the surface, the lower the coefficient of static friction would become. However, it has been found experimentally that when two very smooth pieces of metal have been stacked together, given that all contaminates have been removed and they are placed in a vacuum, they become cold-welded and the coefficient of static friction is very high. It has also been found that two sheets of smooth glass placed on top of each other have a higher coefficient of static friction than two sheets of roughly ground glass (Singh, 2007).
For the case of the coefficient of kinetic friction, F = fk if the block is moving at a constant speed. Once the force is removed that is causing the movement, the block will slow down and eventually come to rest. The only difference between the two coefficients is, once again, whether or not cold-welds have to be overcome. During movement those do not form, so μk< μs.
Lastly, the case of an inclined...
References: Jeschofnig, PhD., P. (No Date). Physics I: Lab Manual of Experiments for the Independent Study of Physics. LabPaq.
Nave, R. (No Date). Friction. Georgia State University. Retrieved on May 19, 2009, from http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html.
Serway, R. (1994). Principles of Physics. Fort Worth, TX: Saunders College Publishing Harcourt Brace College Publishers.
Singh, S. K. (2007). Induced motion on a rough inclined plane.
Connexions. Retrieved May 19, 2009, from http://cnx.org/content/m14077/latest/.
Please join StudyMode to read the full document