lab report 4 - PHY2053L

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Broward College *

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2053L

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Mechanical Engineering

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Dec 6, 2023

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Cristian Acuna Vasquez 01/27/2020 PHY2053L Title: Static and Kinetic friction Purpose: To be able to measure the coefficient of contact between two objects through two unique methods and decide which factors have an effect on such coefficient.. Background Information: The friction forces that arise between any two contacting surfaces are divided into two broad categories based on the relative motion of the surfaces. Static friction exists whenever there is no relative motion and kinetic friction whenever one surface moves relative to the other. The magnitude of each friction force depends on the number of the chemical bonds formed between the two surfaces and their average strength. The direction of each friction force is always opposite the horizontal component of the applied force. Moreover, it can be defined according to the equation: f s = μ s N , where f is the friction force, N is the applied normal force and µ is the coefficient of static friction . When a force exceeds f s , static friction is no longer present. A kinetic friction force f k now acts to opposite the motion. Unlike its static counterpart, f k does not match the applied force up to a maximum value. Its magnitude is constant and given by the formula f k = µ k N . Coefficients µs and µk are depend upon the average bond strength between the two surfaces. The normal force along a flat surface is simply equal to the weight of the object, or N = mg. N is an indirect measure of the number of bonds formed between the two surfaces. In other words, heavier objects require greater forces to get them moving because they press their supporting surface tighter thus forming more bonds.

Materials: Computer, Vernier Computer interface, Logger Pro, Vernier Motion detector, Vernier Dual -range force sensor, String, Block of wood with hook, Balance or scale,. Mass set. Procedure : Part I: Starting Friction To begin this experiment, we measured the mass of the block we would be using throughout the lab. The mass has been recorded in the data table seen below. We then connected the Dual-Range Force Sensor to Channel 1 of the interface and set the range switch on the Force Sensor to 10 N and opened the file “12 Static Kinetic Frict” from the Physics with Vernier folder. One of the end of a string was tied to the hook on the Force sensor, and the other end was connected to a hook on the wooden block. A mass of 0.5 kg was placed on the top of the block . We practiced pulling the block and mass with the Force Sensor using a straight motion. We learned that we would have to slowly and gently pull horizontally with a small face and very gradually increase the force until the block started to slide. It was then our goal to keep the block moving at a constant speed for another second. Part II At this point in the experiment, we focused on measuring the peak static friction force and the kinetic friction force as a function of the normal force on the block. The normal force is a contact force from the surface and is always perpendicular to the surface. Normal force has an equal and opposite force due to gravity. In this portion of the experiment, we pulled the block in the same way we had previously, but we changed the masses that sat on top of the block; therefore, changing the normal force on the block. To start, we removed all the masses from the block and clicked “Collect” to begin collecting data as we had before to gather force vs. time information. Once we had a graph, we examined the data by clicking the “Statistics” button. The

maximum value of the force occurs when the block started to slide. We read this as the value of the maximum force of static friction and recorded the number in our data table. We also examined the region of the graph that corresponded to the block moving at a constant speed by dragging the computers mouse over this region of the graph, clicking the “Statistics” button but this time read the mean (average) force during the time interval. This force is the magnitude of the kinetic friction force. We repeated the above steps for two more measurements and averaged the results to determine the reliability of the measurements. We have 2 more trials with 0.500 kg and 1.00 kg Part III : In this section we measured the coefficient of kinetic friction a second way and we compare it to the measurement we obtained in Part II. Using the Motion Detector we can measure the acceleration of the block as it slides to a stop. This acceleration can be determined from the direction is that of friction. From the mass of the block and its acceleration, we can find the frictional force and the coefficient of kinetic friction.. We set the Motion Detector to “track” and opened the file “12b Static Kinetic Firct” in the Physics with Vernier folder. We then placed the Motion Detector on the lab table and positioned it so that it would detect the motion of the block as it slid towards the detector. We practiced sliding the block towards the Motion Detector. We clicked the “Collect” button and gave the block a push. The velocity graph should have a portion with a linearly decreasing section corresponding to the freely sliding motion of the block. Select a region of the velocity vs. time graph that shows the decreasing speed of the block. Chose the linear section. The slope of the section of the velocity graph is the acceleration. Drag the mouse over this section and determine the slop by clicking the “Linear Fit” button. We recorded this value in a data table. We repeated the previous steps four more times, and recorded those

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values as well. We then placed masses totally .5 kg We then repeated the previous steps three times and recorded the acceleration values in a table. Data and observations: Part I Mass of block 0.065 kg Part II Total mass Normal force Peak Trail 1 Static Trail 2 Friction Trail 3 Average peak Static friction 0.065 0.64 0.494 0.164 0.244 0.301 0.565 5.54 2.175 2.212 2.393 2.26 1.065 10.44 1.938 1.652 2.953 1.179 Total mass Normal force Kinetic Trail 1 . Trail 2 Friction Trail 3 Average kinetic friction 0.065 0.64 0.3111 0.3906 0.1788 0.294 0.565 5.54 2.474 2.462 2.259 2.400 1.065 10.44 2.007 1.515 2.953 2.160 Part III Trial Acceleratio n Kinetic friction force μ k 1 0.5512 0.0358 0.055 2 0.5138 2.85 0.51 3 0.4843 5.06 0.48 average Coefficient Kinetic friction 0.35 Analysis:

1. 2. . The force needed to keep the block sliding is less than the force needed to start the block to slide. 3. Based on the graph the coefficient of static friction should be greater than the coefficient of kinetic friction 4. Total mass kg Normal force N 0.065 0.64 0.565 5.54 1.065 10.44

5. 6.

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7. 8. Trial Acceleration Kinetic friction force 1 0.5512 0.0358 2 0.5138 2.85 3 0.4843 5.06 9 Trial Acceleration Kinetic friction force μ k 1 0.5512 0.036 0.055 2 0.5138 2.85 0.51 3 0.4843 5.06 0.48

10. The coefficient of kinetic energy does no depends on speed. It depends on the property of the surface material. µ k = F/ N 11. The force of kinetic friction depends on the weight of the object. F = µ k N 12. The coefficient of kinetic friction depends only on the property of surface material. It does not depends on the weight of the object or any other factor. 13. Coefficients of kinetic friction determined in Part III and part II are different. Coefficients determined in part II is more precise than Part III. Pulling (Part II) requires less force than pushing (Part III) Error analysis The majority of the sources of error were due observers faults. The person activating the sensor collect button would have some delayed response when the object started to move, thus ithe sensor would record inaccurate readings. Conclusion: In this lab we set out to use a Dual-Range Force sensor to measure the force of static friction, along with a Motion Detector to independently measure the coefficient of kinetic friction. We determined that there is a relationship between the force of static friction and the weight of an object. The higher the mass, the higher the force required to impede motion. We also measured the coefficients of static and kinetic friction for a block and determined if the coefficient of kinetic friction does not depend on weight. Before completing the experiments, I had assumed that weight would play a significant role in both cases of friction. Because of this previous assumption, the results were not what I expected. With my previous understanding, I believed that the coefficient for an object with more mass would be higher. Our data shows that that is not the case. When it comes to the force of kinetic friction, that was shown to be affected by weight

given that in order to find the kinetic force, you had to take the coefficient of friction and multiply it by the normal force, which is calculated by multiplying the mass of the object by the acceleration due to gravity. There could have been some errors in collecting the data as we were trying to pull or slide the block in a straight line. It turned out to not be as easy as it sounded. We collected the data and performed each step to the best of our abilities. This lab helped me differentiate between the force of friction and the coefficients of friction, and helped teach me what they mean individually.

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