Set the block and soup can on a board with the largest surface of the block in contact with the surface of the board. Secure the board so that it does not slide. Place ramp board horizontally on a table.
Weigh the plain wood block and the can of soup. A can of soup will be used to add weight to the friction blocks. Ramp board, 4 feet long set at 10 cm off the ground.įriction block set (one wood, one glass, one sandpapered)ġ. Using the Pythagorean Theorem, the base can be calculated and then used to determine μs. The length of the plank is known, and the height can be measured. For measuring purposes, tan θ = height/base which will make the calculations much easier. Therefore μs = mg*sin θ/ mg*cos θ, or tan θ (Serway, 1994).
The normal force is therefore calculated to be: N = (mg) (cos θ)įrom this diagram, we can see the μs = F/N, where F = mg*sin θ and N = mg*cos θ. The applied force acts perpendicular, normal, to the surface. Recall that the frictional force is proportional to the normal force acting on the block. Lastly, the case of an inclined plane will be examined. During movement those do not form, so μk< μs. The only difference between the two coefficients is, once again, whether or not cold-welds have to be overcome.
Once the force is removed that is causing the movement, the block will slow down and eventually come to rest. 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).įor the case of the coefficient of kinetic friction, F = fk if the block is moving at a constant speed. 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. Intuitively it would seem that the smoother the surface, the lower the coefficient of static friction would become. To calculate the coefficient of friction, the following equation will be used: μs = fsmax/FNįriction is actually a much more complex force than it appears from this introduction, or from the following sets of experiments. The force of friction is in the opposite direction of the movement. If the block is just about to move, then fs = fsmax = μsn. Where μs is the coefficient of static friction. The basic equation for the coefficient of friction is: fs ≤ μsn It is proportional to the normal force, N, acting on the block. As the magnitude of F increases, fs will increase proportionally until is exceeded. If there is an applied force to a block, and the block remains at rest, then fs = F. The first case, the static coefficient of friction, fs, is the force that keeps an object from moving. 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. This is due to the cold-welds formed as discussed above. Unlike most coefficients in Physics, friction behaves differently depending on whether the object is at rest or at motion. Specifically, this lab will calculate the coefficient of friction. 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. Before an object can move over a surface, these cold-welds must be broken. The rough areas of each surface can come into contact and become cold-welded. Various types of materials were used, as well as horizontal versus inclined ramps.įriction occurs when two surfaces come into contact. 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. 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.
In one experiment the wood block was placed on its side and the experiment repeated. 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. 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. A plain block of wood and a block of wood with sandpaper on one side and glass on the other were used. Friction is a force that must be overcome before an object can move across a surface. This experiment measures the coefficient of static friction (μs) and kinetic friction (μk) between objects of different materials.
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