PHYS 4A/ Fall 2014; Coefficients of Friction

Purpose: To use wooden blocks in order  to visualize friction and how it changes under different conditions. In doing this, acceleration of the blocks will be recorded in order to find the friction coefficients.

 This experiment had us determine the different coefficients for different types of friction under various conditions. Therefore, the experiment was divided into five different parts,  each using a different method of applying friction and determining the coefficient. The similarity between the experiments were that wooden blocks were used and were pulled by some force.

For the first part, a pulley system was created for a styrofoam cup and a wooden block. The mass of blocks and the styrofoam cups were measured using a small digital scale. A wooden block of mass 0.1443kg was tied to a string of negligible mass which was attached to an empty styrofoam cup. The set up should look similar to the one below.

Scale used to measure mass of blocks and cup.

Cup and Block setup around pulley

 With the styrofoam cup empty, the wooden block remained at rest as the force in tension was not enough to overcome the force from static friction. Using the knowledge that the block will only move once the force from static friction was overcome, water was added to the cup. As we felt it was closer to equaling the friction force, a dropper was used to add individual drops so that the exact breaking point could be determined. However, the method is still very inaccurate as the block seemed to move within a large range of drops. Once the block had moved, the mass of the cup and water were measured and recorded. Then another block had its mass measured and was added to the stack of wooden blocks. The experiment was done again until the block moved. This was done a total of 4 times in order to obtain values up to a stack of four wooden blocks which were recorded in excel. Using these values, the friction force was calculated using Newton's Second Law and the friction force was compared to the normal force. Using the equations below, the friction coefficient was determined.

By inserting values into excel, we were able to quickly calculate the values for friction force and normal force. With this info, we could graph the relationship between the two.

As you can see, there is a direct relationship between static friction and the normal force which makes logical sense as the heavier something is, the harder it is to move. 

For part 2 of the experiment, a force sensor was connected to the wooden block in place of the cup. The sensor was then dragged across the table and LoggerPro was able to tell us the force of the pull from the sensor to the block. Again, measurements were recorded until a total of 4 blocks had been stacked and recorded. 

                                                                                Once a pull was recorded, if there was a pretty constant velocity, there would be an area where the force also is constant within a small range. The mean force within this interval was taken in order to determine the force used to pull the block.

Using a similar method as part 1, the kinetic friction force and normal force were calculated and graphed in excel.

Again the kinetic friction force and normal force were found to have a direct relationship which makes logical sense.

Part 3-5 again found the friction forces of the block to its surface, however this time the block was set on top of a metal track. For part 3, the block was set at some point on the ramp where it would remain at rest. Then ramp was then lifted in order to increase the angle until the block started to move. The maximum static coefficient was determined for this scenario again using Newton's Second Law and Free-Body diagrams.

Using the equations below, the friction and normal force were able to be calculated. At an angle of 20 degrees, the friction force coefficient was determined to be 0.364.

For part 4, the same thing was done with a steeper incline meaning that the friction calculated would be kinetic friction. At an angle of 25 degrees, using the equations below, the kinetic friction force coefficient was calculated to be 0.355.

For part 5, the experiment was done in the opposite direction. Instead of having the block slide down, we had it slide up the ramp by attaching the string to a hanging mass. The hanging mass was put on a pulley and the kinetic friction coefficient was calculated at three different angles.

Using a motion sensor at the bottom of the ramp, we were able to measure the acceleration of the block moving up the ramp. As we knew the angle of the ramp, we were able to again use Newton's Law to determine a kinetic friction coefficient. Using the first two trials to determine a friction coefficient of 0.2, we were able to make a theoretical guess for the acceleration of the 3rd trial. However, after doing the calculations, we realized there was a % difference of 500% between our theoretical value and the calculated value. This was a very high number, however as the experiment was very inaccurate and we understood the concept, we moved on.

In conclusion, the experiment allowed us to calculate different static and kinetic coefficients under different situations. We used Newton's Second Law and were able to find values for each experiment, even if some did seem to be off.