Friction is Fun!!
The lab starts out in a classroom. Ideally, the students have not yet covered friction in the lecture section. A brief discussion is made on friction by using inquiry methods to get the students to figure out what is the direction of friction by using a cardboard box with a string attached to the box. This string is then connected to a newton-scale. The box is then pulled along at a constant velocity. At this time it is pointed out that in all the experiments to be done that it is important to keep the string and scale parallel to the table-top. The concept of kinectic frictional force is then discussed along with a free body diagram of the moving box. Then the students are asked what the kinetic frictional force may depend upon. They usually answer with 1). Texture of the surfaces. 2). The weight of the box. 3). the surface area of the box. Then the instructor asks the class to come up with an experiment to test the effect of weight on the kinetic frictional force. They come up with measuring the frictional force as the weight of the box is varied then fit a trenline to the data. Next they are asked to come up with an experiment to measure the effect of surface area on the kineteic frictional force. They then say do the same experiment as done previously only now put the box on a different side which has a much different surface area than the first experiment. We don't test texture of surfaces since that is kind of obvious. We then go into the lab with each group performing both experiments. Once each group has finshed and fit trendlines to the data they are asked to write down the slopes for both their graphs. Usually groups are amazed at how similar the values for the slopes obtained are. We then go into the classroom and each group writes the value of their slopes obtained for the large surface area and for the small surface area on the chalkboard. This allows the class to see that all groups have similar results. The mean and standard deviation of the mean are then calculated for each surface area and generally both values agree! The we discuss the linear trendline fit as being y = mx +b. The data plotted was with the kinetic frictional force, f, being equal to y and the weight equal to x and then the slope parameter is discussed and it is explained that this is the coefficient of kinetic friction. Going back to the free body diagram it is discussed that the magnitude of the weight is equal to the normal force N. It is discussed that we can ignore b in the above equation. So we then obtain the equation that
f = uN
This lab works wonderfully, the students really like seeing that a lab so simple can give so much information.