6. Friction, Experiment and Theory


The lab this week investigates the frictional force and the physical interpretation of the coefficient of friction. We will make use of the concepts of the force of gravity, the normal force, the frictional force, tension, free-body diagrams, and objects sliding on an inclined plane. Recall what you know about friction before coming to the lab.


Pre-Lab activity

Please complete this activity before coming to the lab session and submit your results through Moodle.

To study the question of friction theoretically consider the online simulation which represents two blocks connected by a string, where one block is located on the table and another block is hanging from the table. The simulation allows changing mass of each of the blocks as well as the coefficient of friction between the block and the table. Try various values for masses and friction coefficients, see what happens. Notice how acceleration of the system changes as you change these variables. Determine the unknown values for masses and coefficient of kinetic friction.

To resolve this problem as well as to perform calculations for the real experiment, which you will perform in the lab, you have to be familiar with how to apply Newton’s second law for this case.


Lab Experiment

1.      The experimental design for the first part of the lab will be very similar to what you saw in the online demonstration. You will be using a block connected by a string with an additional mass hanging from the table. You should also set up the motion sensor in such a way that it allows you to measure the speed of the block. This way you can determine whether or not the block is accelerating as it moves along the board. Using this experimental setup with a horizontal surface, figure out which quantities you should plot on the graph in order to determine the coefficient of kinetic friction for the block sliding on the board.  Hints may help to figure this out.

The provided blocks of wood have hooks for pulling and have various materials attached to them to vary the surface in contact.  Figure out how to measure the normal force and the frictional force while the block is sliding across the board. See Hints for help figuring out how to do this. Vary  (for at least five values) and measure the corresponding  for a given block.  Make a plot and have Excel calculate the coefficient of kinetic friction from this graph.

Repeat for two more blocks. Include all three plots on the same graph (put a legend on the graph). Which surface has the highest?  Does that make sense?


2.      Set the surface at an angle, , from the horizontal. Place the object on the surface and increase the angle until it slides.  The angle at which the object just begins to move is defined as the “angle of repose”.  Figure 1 illustrates this situation.  From the diagrams, it can be shown that there exists a relationship between the angle of repose,, and the coefficient of static friction, . Determine this relationship and solve for. Note that in your case the block is just about to start moving so it has no acceleration yet. However, to solve this problem you may also look at the more general situation when the block is already moving.

Using the same blocks as before, compare your results with those for coefficients of kinetic friction, determined in Step 1.








Figure 1. Finding the angle of repose.



3.      Select a block and an angle. Compute the force required to slide (pull) the object up the inclined plane (See Fig. 2) using the values of  determined in Steps 1 and 2. Carry out the experiment and compare (via %-difference) the experimental value with the theoretical value for that type of material. Once again you can use the motion sensor to control whether or not the block is accelerating. See Hints for help.








             Figure 2. Sliding up an incline.


4.      If you still have some time left try to design and test an experimental procedure to determine the effect of surface area and velocity upon the coefficient of friction of wood-on-wood.  Discuss your results.