Physics 4A (9/21/16) Friction Forces

Physics 4A (9/21/16)

Friction Forces

Brett Mccausland

Lab Partner: Olivia Phuan

Purpose

Model frictional forces in five different experiments, determining the effect that both static friction and kinetic friction have in several different scenarios. Use experimental data to calculate friction coefficients.  

Procedure (1) Static Friction

As depicted below, we set a painted wood board on top of a flat table and held it in place with a clamp. At the end of the table we put a pulley, and ran a sting over it with a mass holder hanging from one side, and the other side of the string was tied to a wood block with linoleum surface. We measured the masses of the wood blocks seen in the photo.We ran 4 trials, each trial we slowly added masses to the hanging mass holder until the point at which the wood block broke static friction and began to move. The first trial we only had the single block however every trial we added one more wood block on top of the wood block on the board to increase the normal force between the board and the block.We performed this procedure in order to gather data on static friction coefficient between the board and the block. By gathering data on the mass required to set the system in motion we will be able to calculate the force required and then find the static friction coefficient.  

Procedure (2) Kinetic Friction

For this experiment we used a force sensor to read the force applied in the tension of the string tied to the block while it was being pulled at roughly a constant velocity.Like previose experiment we ran 4 trials, increasing the number of blocks evertime. We did this in order to collect data on the force required to keep the block at a constant speed.This data on the force will allow us to find the coefficient of kinetic friction

using newton's second law.

Procedure (3) Static Friction from a sloped

For the third procedure we placed the block on the wood board as well as a smart phone with the ability to read the angle of the board.We slowly raised the board until the point in which the wood block broke static friction and recorded the angle in which this occurred in order to latter calculate are static friction

coefficient

Procedure (4) Kinetic From Sliding Block Down Incline

For this procedure we taped a piece of paper to one end of the wood block and slightly raised the angle of the board from the previous experiment.We did so because for this experiment we wanted to gather experimental data on the block’s acceleration down the plane with the use of a motion sensor.The paper made it easier for the motion sensor to detect the movement of the block and the incline was increased in order to increase the magnitude of the force down the board in order to overcome the force of static friction, and put the block in a state in which its under the force of gravity and kinetic friction.

Procedure (5) Predicting the acceleration of a two mass system

For the final procedure we first made are calculations for what are experimental kinetic friction between the wood board and the wood block was from are previous data.Then based on the mass hanging, the mass of block and the kinetic friction coefficient we found, we made a prediction on what the acceleration would be when the system was put in motion. The system as seen in the image was the block on top of a wood board, with a string attached to it, that went over a pulley, and had a mass attached to the other end suspended off the edge of the table.With the use of the motion sensor like previous experiment we found are experimental acceleration and made a comparison to are calculated theoretical acceleration.

Calculations (1) Static Friction

In order to find the coefficient of static friction for the block given the data we collected I first drew a force diagram for the sum of the forces in the x direction and the sum of the forces in the y direction. Since we were taking static friction the sum of the forces in the x and the y direction are equal to 0. I solved for static friction and then made a excel spread sheet with my data using the formula I found for static friction . The I found the average of all for trails .

Calculations (2) Kinetic Friction Force Sensor

Like the previous experiment I first made a force diagram, since for this experiment I had a force reading and the block was pulled at a constant acceleration the net force was equal to zero. Therefore I divided the force reading by the natural force to solve for the coefficient of kinetic friction. After I found the formula for Kinetic friction coefficient I entered in all the data into excel to calculate the kinetic friction coefficient for all the trails as well as get a average.  

Calculations (3) and (4) Static and Kinetic Friction From Sloped Surface

The next two experiment were done on an incline, experiment 3 was static friction on an incline and the data we had was the mass of the block and the angle and we wanted to solve for the coefficient for static friction. I did so by first drawing a force diagram, due to the fact that the block was not moving in either direction the sum of the forces in the x and the y direction were equal to zero.  Solving for natural force and the for due to gravity and I arrived at the sum of forces seen in the image of the calculations .I then moved the force of gravity over and divided by the normal force to isolate static friction coefficient and from there we plugged in are experimental values and found a experimental coefficient for static friction of (0.426). For experiment (4) I used the force diagram I had drawn in the previous experiment. For this calculation we wanted to solve for the value of are kinetic friction given the angle of the board and the acceleration of the block. Since the block was in motion there was a net force equal to the mass of the block and its acceleration that we found from are experiment. Therefore for this calculation I solved for the kinetic friction by first solving for the natural force and the force due to gravity and set them equal to the net for on the block.Then solved for kinetic friction coefficient that I found to be 0.135. See image for full calculations.

Calculations (5) Predicting the acceleration Of a Two Mass System

We began the calculations for the theoretical acceleration that we expected using the average kinetic friction that we found from experiment (2). As seen in the image of the calculations I began by drawing a force diagram and solved for acceleration. Using the average kinetic friction coefficient and the mass of the two objects I found a theoretical  acceleration of 1.4 m/s^2. The experimental acceleration was 1.2 m/s^2 a

difference of 0.2 m/s^2 and a error of 14.28%

Conclusion and Data Analysis

For the first experiment the accuracy of the static friction was only as accurate as the amount of weight that was being added at a time.For instance if every time the block didn't break free you added 20 g then if it broke free on the next mass added than the accuracy would be (+/-) 20 grams which would not be very accurate.For us do to lack of time and supplies the mass was increased at 5 gram increments therefore the accuracy of the static friction is +/- 5 grams.Also there was a good deal of possible random error in the data since at different places on the board there was slightly different texture to the board that the block was sitting on. For Kinetic friction experiment 2 the angle at which the block was being pulled was very important since in the calculations we were taking the force read as the magnitude of the force in the x direction so any slight angle to the pull would cause error. Also with experiment 2 the pull velocity was very error prone since there needed to be no acceleration since we were setting the sum of the forces equal to zero any acceleration could effect this data and it was difficult to be consistent using only human accuracy.The third experiment for static friction on an incline the greatest variable was the texture of the surface, it had proven to be the greatest inconsistency of the experiment. The kinetic friction on an inclined board was of all the experiments the least error prone since there was very little external influences once the block was put in motion. The block did experience unaccounted for air friction especially since we added a large flat surface to the back of the block for the motion sensor reading. Overall though since the block never reached a very high velocity it is still negligible for are purposes. The prediction experiment for acceleration as mentioned had a experimental error of 14%. After evaluating the data the most consistent friction coefficient was static friction. The first value for static friction that I found was on the mass pulling the block which was (0.478) and the second value for static friction that I found was  (0.425). In comparison the first value found for kinetic friction was (0.23) and the second value for kinetic friction was (0.135). It does however make sense that we were not able to calculate kinetic friction as accurately as we were for static friction since static friction is a much larger force it takes less precision and will have less percentage of the measurements be in the range of propagated error.