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Homework answers / question archive / PHY171 Laboratory 5 Friction Materials: Paper Grocery Bags String Hardback Book(s) Background In this lab, we will investigate the phenomenon of friction

PHY171 Laboratory 5 Friction Materials: Paper Grocery Bags String Hardback Book(s) Background In this lab, we will investigate the phenomenon of friction


PHY171 Laboratory 5 Friction Materials: Paper Grocery Bags String Hardback Book(s) Background In this lab, we will investigate the phenomenon of friction. Friction is the force that resists the motion of two objects sliding against each other. You probably already have some good intuition about friction. For example, we can imagine that it is harder to slide two rough objects past each other than two smooth ones. Friction is not a fundamental force like gravity or electromagnetism, but is instead caused by the microscopic imperfections on the surfaces of objects. When objects slide past each other, tiny pits and bumps catch on each other making it harder for the objects to slide. We can reduce this catching by smoothing out the pits and bumps on surfaces. As we mentioned in lecture, the force of friction depends greatly on what the objects that are sliding against each other are made of. It also depends on how hard they are pressed together. Consider the figure below where mg is the force due to gravity on the object (also called its weight), Fa is the applied force, Ff is the force due to friction and FN is the normal force. FN Ff Fa mg The applied force is to the right, so the opposing force due to friction is to the left. This opposing force is proportional to the normal force Here µ is a unit-less scalar that is called the coefficient of friction (COF). For this particular case, we can easily see that the normal force must be equal to the objects weight (if we had an additional force pulling upwards, the normal force would be less). Thus, the force due to friction depends on the COF and how much force is transmitted down on the bottom object. The COF is different for different materials and is usually between 0 and 1, but for some material (like rubber sliding on rubber) it can be greater than 1. You may have noticed when sliding a heavy object across the floor that it is hard to start it moving, but once it is in motion it seems easier to push or pull. This is in fact the case. For objects that are not moving, µ=µs, the coefficient of static friction. For objects that are moving, µ=µk, the coefficient of kinetic friction. In general, µs≥µk as you will verify in this laboratory exercise. Activity 1: Static Friction Friction is what tends to hold things in place. We’ve learned in class that the force due to friction follows some basic rules. 1) ?? = ?? 2) There are two types of friction, static and kinetic. 3) The force due to friction always points against the motion, or the attempt to move something. Prep the surfaces In order to measure friction, we will need two surfaces that rub against each other. We will accomplish this by wrapping a book in the material from a paper grocery bag, and sliding it on a sheet made from a second paper grocery bag. Ideally, you’re looking for a hardback book (as they’re generally heavier). Use your scale to determine its mass. Make the appropriate conversions as necessary. mbook= ± kg Wbook= ± N You also will need a small bag, such as a ziplock bag or plastic grocery bag. This bag will be used to hold convenient weights (such as marbles, coins, or silverware). The weights should be small enough that adding a single item won’t increase the weight that much, but as a group is comparable to about half the heft of your book. Take a length of string, and tie loops at either end. Slip this into the middle of the book to form a tow string that reaches about 3-4 feet from the book. Cut the paper grocery bag apart so that you can wrap the whole of the book, passing the paper through the triangle formed by the tow string. Insert one end of the paper into the book between the cover and inside first page. Wrap the paper bag tightly around the outside of the book, and insert the other end between the back cover and last page. 1) Cut the bottom off the bag. 2) Cut the loop open at the overlapping seam along the side of the bag. 3) Trim as needed. Cut a second bag apart to create a long flat sheet with no seams. Tape the sheet to a table. Arrange everything on the tabletop, adjusting the positions of everything as needed. Your goal is to simultaneously meet the following goals. 1) The book is placed at one end of the taped long sheet of paper bag material, with the string reaching across the length 2) The string is tied to the small bag (for weights), such that the small bag is suspended just over the edge, as high as possible above the ground. Collect Data Slowly add weight into your small, suspended bag. At some point, the book will suddenly start to slide, so be ready to catch everything. Use your scale to measure how much mass was required for the book to start sliding. mbag= ± kg Wbag= ± N From the information you have gathered, determine the coefficient of static friction for grocery-bag-on-grocery-bag Repeat this activity at least three times, and average your results. ?s= ± Activity 2: Static Friction, redux Select a second book, and determine its weight. mbook2== ± Wbook2= ± kg N Add this book on top of your previously-wrapped book and repeat the experiment you completed in Activity 1 (but using both books, instead of one). ?s2= ± Think About It: Does this answer make sense compared to the Activity 1 results? How? Activity 3: Kinetic Friction Now we get to look at kinetic friction, the friction that applies when things are moving. Think About It: If all forces on an object are equal and opposite, what should the resulting acceleration be? Place your single book and string combination on the same table as in Activity 1. As before, add weight to the small bag, but each time tap the book slightly in the direction it would want to move. Think About It: Why do we tap the book? Once you find a weight when the book can be “encouraged” to move, note the motion. Finely adjust the weight so that the book travels across the table at a constant velocity. mbag== ± kg Wbag== ± N Repeat this activity at least three times, and average your results. ?k= ± Think About It: Does the behavior of the coefficients of static and kinetic friction conform to your understanding from lecture? How so? Deliverables We need an informal report describing what you did and what you learned. Imagine you are talking to your parents or your boss, and describing the activities you just completed. Make sure to include any pictures and resulting understanding you have gained. Submit a copy of this report and your video file for grading. Rubric: Documentation Missing 25 (0.00%) Novice 15 (60.00%) Partial 20 (80.00%) Proficient 25 (100.00%) Did not submit Narrative unclear, incomplete thoughts and/or sentences. Did not include sufficient information for a person to replicate the work Narrative was fairly clear, but left out something significant (i.e, meaning of the results, numbers without units or uncertainties) Ideas were expressed in a clear and organized fashion. It was easy to figure out what was going on, and how to repeat the experiment if desired. Included discussion of results compared to accepted values (with appropriate uncertainties and units) A post-lab quiz will also be required to assess your understanding of the goals for this lab, and will count for half the grade. Friction is a force that resists the movement of objects in contact with one another. This force acts in a specific relationship to the direction of motion or attempted motion. Which way is it? Opposite The same The direction of an applied push The direction of an applied pull Question 2 (1 point) Does the coefficient of friction (u) depend on how much mass an object possesses? Yes No Why is it necessary that the book moves at a constant velocity to measure the kinetic coefficient of friction? AJ Question 4 (1 point) Explain in regular words what the coefficient of friction is. Explain the difference between static and kinetic friction. Give multiple real life scenarios in which it is important to know the coefficient of friction. State a few ways to increase frictional force, and a few ways to decrease it, in those real life scenarios.

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