Group6_6_ch4

=Absentee Wikipage= toc

__Lab: Coefficient of Friction __
12/5/11 Joey Miller, Ryan Hall, and Caroline Braunstein

- To measure the coefficient of static friction between surfaces - To measure the coefficient of kinetic friction between surfaces - To determine the relationship between the friction force and the normal force
 * Objectives: **

**Hypothesis:**

- The coefficient of static friction between surfaces will probably be closer to 1. This is because when the coefficient is greater than or equal to one it implies a very rough surface, this is needed for static friction to occur. - The coefficient of kinetic friction between surfaces is closer to 0. This is because the lower the coefficient is the less friction and the more sliding that will ultimately occur. - Normal force and Friction force are directly proportional to each other, which means that if the Normal force on an specific object is high, so will the amount of friction force and vise versa.

**Video:** media type="file" key="Static-Kinetic Friction.mov" width="300" height="300"

The lab set up looks something similar to this.
 * Picture: **

**Materials and Methods:** For this lab, we will be using a Pasco force meter, a USB link to connect it to a laptop, a friction 'cart,' miscellaneous masses, string, an aluminum track, and a clamp.

First, we will find the mass of the friction dynamic cart, and then place the cart on the track, with a 500g mass in it. We will add a 15cm string to one end of the cart, and on the other end, the Pasco Force Meter. When we insert the Force Meter USB link into a laptop, we will choose DataStidio, click "New Experiment," "SETUP," check "Force-Pull Positive," and uncheck "Force Push Positive." On the display of the graph, we will change the y-axis to "Force Pull Positive," and press the zero button on the sensor. We will then press "Start" on DataStudio, and gently pull the force sensor with a constant speed parallel to the surface, which will in turn pull the cart. For the graph, we will highlight the section of the straight line, click the symbol representing net force, and then select the mean as the value for tension and constant speed. Next, we will highlight the maximum point and record it as maximum tension, and then repeat twice for each mass, and repeat each step for larger masses.

**Data Table:** __Static Friction:__

__Kinetic Friction:__

__Class Data:__



**Graph:**



**Calculations:** __Free Body Diagram:__

__Sample Calculation of Tension and Friction:__
 * Because the cart is moving at a constant velocity (once it is set into motion), there is no acceleration on the x-axis. This tension is the average tension, for the specified mass (0.593 kg).

__Sample Calculation of Normal force and Weight:__
 * The acceleration on the y-axis is zero because there is no movement on the y-axis.

__Sample Calculations for Coefficient of Static Friction (μ):__ __Sample Calculations for Coefficient of Kinetic Friction (μ):__

Compare the slope of line with calculated μs average (% difference). __Static Friction:__ __Kinetic Friction:__ Compare your result with the class results. __Static Friction:__ __Kinetic Friction:__ **Discussion Questions:** //1. Why does the slope of the line equal the coefficient of friction? Show this derivation. // 
 * Analysis:**

//2. Look up the coefficient of friction between your material and the aluminum track. Discuss if your measured results fall within the range of theoretical values. Be sure to cite your source! // For Static friction the coefficient of friction is supposed to be between 0.25 and 0.4. However our coefficient of friction was slightly lower at 0.2025. For kinetic friction the coefficient of friction is supposed to be between 0.1 and 0.3, and our coefficient of 0.1275 which is between the range it is supposed to be. Our source was [].

//3. What variables affected the magnitude of the force of friction? What variables affected the magnitude of the coefficient of friction? // Friction is equal to weight which is mass*gravity. The weight of the cart as well as the coefficient of friction affect the magnitude of the frictional force. The frictional force and the weight of the cart affected the magnitude of the coefficient of friction during this lab.

//4. How does the value of coefficient of kinetic friction compare to the value for the same material’s coefficient of static friction? // Our coefficient of static friction is greater than the coefficient of kinetic friction by 0.075. This is because when an object is not in motion the frictional force acting upon it is stronger than when the object is sliding. The static friction was the maximum tension during the lab to get the object moving thus it will be larger than the coefficient of kinetic friction. The website we referenced above also confirmed this...

**Conclusion:**

Our hypothesis were fairly accurate. We assumed that static friction would be greater than kinetic friction. In reference to static friction we assumed it would be close to one. It was larger than kinetic but was not that close to one. We also assumed that kinetic would be close to 0 which it was. We knew that one of the slopes would be small because of our understanding of the µ coefficient. We were told that the value of µ has to be between 0 and 1. This knowledge led to our establishment of these hypothesis. For static friction we got a coefficient of .2025. For kinetic friction we got a coefficient of .1275. These are small, we expected the kinetic to be small but the static was much smaller than we had predicted. In reference to error there are many potential sources. One source of error was if we didn't pull the string exactly horizontally it could have thrown our results off. Slight variation in movement by ones hand could have caused the sensor to compile slightly changed data. We also could have thrown the data off slightly by pulling the string too quickly causing unnecessary acceleration. These sources of error would be small but ultimately that account for the slight variations that we saw. If we could change the lab to make is better we would have done a few things. The first thing was to make sure that the string is perfectly parallel to the ramp at all times. We would also have tried to have a device pull the cart rather than our hands. If we did these two things we would significantly reduce the potential for error in this laboratory. This can be applied to real life in a few circumstances. One example would be a group of people pulling something heavy like a refrigerator. If the people are pulling the object up their driveway they need to know the coefficient of friction in order to determine how many people they are going to need to help them pull it. This can also be applied to scenarios like a tow truck and a tire to road contact.

=LAB, COEFFICIENT OF FRICTION= Ben Sherman (A, B, C)

-To measure the coefficient of static friction between surfaces -To measure the coefficient of kinetic friction between surfaces -To determine the relationship between the friction force and the normal force.
 * Objectives:**

I hypothesize that the coefficient of both static and kinetic friction will be between zero and one. I also hypothesize that the relationship between the friction and normal forces will be directly proportional.
 * Hypothesis:**

In this lab, we used a force sensor that connects to a Pasco USB link, a friction "cart", miscellaneous masses, string, aluminum track, and a clamp.
 * Materials:**

First, I recorded the mass of the cart, then I placed the cart on the aluminum track, which was on a table. Once I did this I added a weight to the cart and then I tied a small piece of string to the cart, and then tied the string to the force sensor. Once the force sensor and cart were set up, I plugged the sensor into the computer, entered Data Studio, and selected "Create Experiment." In Data Studio, I selected setup, unchecked force-push positive, and selected force-pull positive. On the graph, I clicked the label on the Y-axis, and changed it to Force - Pull Positive. After Data Studio was configured, I left some slack on the string, pressed zero on the force sensor, and clicked start in data studio. Once I collected my data, I ended the experiment, and selected the straight line part of the graph that I wanted more information on. I clicked the sigma, then selected record MEAN as the value for tension at a constant speed. I then highlighted the maximum point, and recorded this value as the maximum tension. After each of these trials, I would change the mass in the cart and repeat the data logging procedure ad record the new data.
 * Methods:**


 * Data:**

ANALYSIS:
 * Graph:**

FBD For the Cart:
 * NOTE I was given old data, and was thus not able to get a class average to do percent difference and percent error**

Sample Calculations: 2.**)Look up the coefficient of friction between your material and the aluminum track. Discuss if your measured results fall within the range of theoretical values. Be sure to cite your source!** Can't complete question. I was given data for cork on aluminum, yet several pages of google searching was unable to bring up the coefficient of either static friction, kinetic friction, or both for cork on wood.
 * Discussion Questions:**
 * 1.)**

In this experiment, the normal force of the track acting on the cart was equal to that of weight. Weight is the object's mass multiplied by g, which is always 9.8. This means that whenever the weight changes, the so did the normal force of the track on the cart. Since friction = mu * normal, and the coefficient of friction for my materials, cork on aluminum is dependent on the friction force and the normal force. If the coefficient of friction was to stay constant, than the force of friction would have a positive correlation with the normal force, in that whenever one force increased or decreased, so did the other. In the case of friction, the variable affecting the magnitude was the weight of the cart, and the amount of weight in the cart. The coefficient of friction is affected by the force of friction and the normal force (weight). If the weight were to increase and the force of friction were to stay constant, the coefficient of friction would have to decrease, and if the weight decreased, the coefficient of friction would have to increase. If weight were to stay constant, and friction were to increase, then the coefficient of friction would have to do the opposite of the change in friction to counteract the change.
 * 3.) What variables affected the magnitude of the force of friction? What variables affected the magnitude of the coefficient of friction?**

The differences in kinetic and static friction for cork on aluminum are significant. The average coefficient of kinetic friction was .563. while the average coefficient of static friction was .647. This is a difference of .084, which is large, given that most coefficients of friction are between 0 and 1. This data makes sense, because static friction is always larger than kinetic friction for any given material, because there's a higher force needed to move an object from rest than to keep it in constant motion, because the coefficient of friction is smaller for kinetic friction.
 * 4.) How does the value of coefficient of kinetic friction compare to the value for the same material’s coefficient of static friction?**

The objective of this lab was to find the coefficient of static and kinetic friction between surfaces (in my case, cork and aluminum) and to determine the relationship between friction and the normal force. I hypothesized that both static and kinetic friction will be between zero and one, and that the relationship between between the friction and normal forces are directly proportional. After experimenting, I have found that my hypothesis is correct. From the data, I found that average coefficient for static friction was .647, and for kinetic friction, it was .563. Both these values are clearly between zero and one, proving the first part of my hypothesis correct. For the second part of my hypothesis, the graph clearly illustrates that I was correct. For both static and kinetic friction, the friction force v. normal force graph shows that the line has a constant positive increasing slope, and shoes that as the normal force increases, so does friction, and vice versa.
 * Conclusion:**

This lab did have several sources of errors that could be fixed. One of these error points is when the force sensor is pulled for the first time, if the puller isn't gradual and applies little force first, and yanks at it instead, it could cause a spike in the data, leading to incorrect values. This could be fixed by making sure (or putting in the lab sheet) that the person pulling the force sensor starts out with very little pulling. Another source of error was that the track/table could be at an angle, causing the cart to have to go up at an angle, or do down at an angle. If the cart had to be pulled up, it would require more force, causing inconsistent data, and if it were sliding down at an angle, it would also cause inconsistent data. This can be prevented by checking the track with a level, to make sure that it isn't angled.

The coefficients of static and kinetic friction in this lab can be applied to everyday life. One example of these principles being for the design of cars and their tires. This is because the coefficient of friction between rubber tires and the road changes depending on if the road is wet or dry. Cars and tires have to be designed so that they won't spin out on different road conditions, such as when the road is wet, or muddy, or snowy. For example, coming out of a turn would mean a changing coefficient of static friction. The car could also be designed so that when driving at a constant speed, the tires act correctly on different road conditions and are safe to drive with.