Physics 1 Lab report

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Friction Equipment: wooden block (with leather underside), board dual range force probe, motion detector, Logger Pro software Newton weight set, string Friction1 is the term we use to describe the force that acts between two surfaces in contact along a direction parallel to the interface of the surfaces. Frictional forces are passive forces in that they only arise in response to the action of other forces. This is in contrast to a force like gravity, for example, that acts as a result of the proximity of two massive objects. Place a block of wood on a table surface. If the surface is level the block will remain at rest and no friction force will act on the object. Incline the surface or pull on the block and the block will still remain at rest up to a point. A frictional force is acting on the block (exerted by the table) that resists the relative motion of the two surfaces (underside of block and table surface). This is called static friction. You know from everyday experience that with enough force the block will start to slide across the table. Once this occurs relative motion exists between the surfaces. We know that friction is still acting as without the application of a sufficient amount of force along the direction of motion the block will slow down and stop. When there is relative motion between two surfaces in contact the resulting frictional force is referred to as sliding (or sometimes kinetic) friction. Activity I 1. Connect the force probe and motion detector to the computer at your lab station. More than likely both probes will connect to the computer’s USB ports. 2. Start the Logger pro software. Logger Pro should generate three graphs: position, velocity and force all versus time. Delete the position graph (select the graph with the mouse and use “cut” from the Edit menu). Select Auto Arrange from the Page menu to “pane” the remaining two graphs so they fill the graphing area on the screen. Set the Select Data Collection from the Experiment menu and set the length of the experiment to 10 s. A range2 on the force scale from 0 to 5 N should be sufficient for now. The velocity scale will need to be small, perhaps 0 to 0.1 m/s for now. Adjust graph scales as needed.

The diagram above describes the physical arrangement of the experiment. A wooden block is placed on a board and the force probe is attached to the block. The motion detector is placed on the board so that it can track the motion of the block.

1 See Introductory Materials, Sections 1 and 2 in regard to data collection and analysis. 2 Changing the axis scale in Logger Pro is easy: just click on the max/min values on the graph and type in a new value.

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Note: One “wide” side of the block you will use for this activity is bare wood. The other is covered with a strip of a substance that either is leather or resembles it. Use one side of this block for activity I. In activity II use the other side. 3. To collect data click on Collect and then begin pulling on the block; gently at first but gradually increasing the amount of force. The block should remain at rest for several seconds while you are applying force. Continue pulling after the block begins to slide, and pull in such a manner that, once in motion, the block slides at (nearly) constant speed. Check with the instructor then on the axes below make a sketch of your force-time and velocity-time graphs.

Question(s) How can you tell that motion has begun by looking at the velocity-time graph? Mark the graph to show the point where motion begins. Describe what happens to the applied force at the point where motion begins.

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Prior to the onset of motion is the amount of force applied constant? How can you tell? If not, describe how this force varies. Below is a free-body diagram of the block. The vertical force labeled Ftable is sometimes referred to as the “normal force” as it acts perpendicular to the surface of the table. Whenever two surfaces are in contact and pressed together compressive forces act on each surface perpendicular to the interface between the surfaces. In this case the weight of the block, Fg, produces the compression. The normal force is a passive force while gravity is not. Horizontally the friction force (also passive) acts opposite to the applied force.

How should the normal force compare to the force of gravity (the weight) during the experiment you just performed? Explain your reasoning. While the block is at rest how should the friction force compare to the applied force? Explain your reasoning. What must be true about the motion of the block if the applied force equals the friction force when the block is in motion?

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4. Use Analyze/Examine to determine the maximum amount of static friction exerted on the block from the data you collected in step 3 (i.e. the amount of frictional force acting right at the peak of the force-time graph). Static Friction Maximum ________________ N 5. Also using Analyze/Examine, drag the mouse pointer over a section of the force data taken while the block slid at constant velocity. Use Analyze/Statistics to determine the average frictional force exerted on the block over the selected interval. Average sliding friction force _____________N Use Experiment/Store Latest Run to move the data you have collected to the background. You will need this data for comparison later. 6. The force vector labeled Ftable often called the “normal force” in the free body diagram above can be thought of as describing the degree to which the two surfaces are pressed together. Describe how you could determine the magnitude of this force for the experimental situation in step 3. Make the appropriate measurements and state the result below. Normal Force ________ N 7. Place a 1 N weight on the block and repeat the data collection procedure of step 3. Keep this data on the screen. Question(s) What is the magnitude of the normal force for the measurements you just performed with the 1 N weight on the block? Based on your measurements do either or both the static friction maximum and the sliding friction force appear to depend on the normal force? Explain how you know.

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8. Design an experiment using the data collection process from step 3 that explores the relationships between both the normal force and the static friction maximum and the normal force and the sliding friction force. Collect enough data3 over a wide enough range to produce graphs that illustrate any mathematical relationships that may exist between the variables. Make a table(s) below to list your data and label the table(s) to indicate whether you are using the wood or leather side of the block. Plot graphs, using the Graphical Analysis software, with the normal force as the independent variable (on the horizontal axis). Label the graphs appropriately so they are easy to identify. Question(s) From your graph describe (in words) the mathematical relationship (e.g. linear, nonlinear...) between the normal force and the static friction limit. Repeat for the normal force and the sliding friction force. Allow for scatter in the data. If we attempt to model the data on each graph with linear functions then in each case we have:

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f = aFN + b where f is the friction force, and FN the normal force. The constant a is the slope of the line. b is the intercept.

3 The more data you collect the better. Interpretation is easier, and there is less need to go back and figure out what happened with this or that data point. Within reason collect as many data points over as wide a range as possible.

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What are the units of the two constants in the equation above? Explain how you know. When linear equations are discussed in general the form

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y = ax + b is often used. How is the slope interpreted? How is the intercept interpreted? In terms of the equation

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f = aFN + b how should the slope and intercept be interpreted? Does it make sense that the intercept of this linear function, given the meaning of the variables, be nonzero? Discuss. A student studying friction produces graph A below from his data and concludes that the trend in the data is nonlinear. Accordingly he adds a nonlinear trend (a curve) to the graph to describe the data. To be sure he collects another data point and draws graph B using the extra data and the data from which graph A was produced. Was the student’s conclusion regarding the nonlinear trend justified? Discuss.

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9. Using the best fit feature of Graphical Analysis (Analyze/Curve Fit or Analyze/Linear Fit) Fit the data on each of the graphs to linear functions. Write your best fit equations below using correct variable symbols (not x and y) and include correct units for all constants. Print the graphs and include them with your lab report. Question(s) Does a linear model describe your data? Discuss. Recall your interpretation of the intercept for the friction force-normal force relationship. Do your best fit lines have nonzero intercepts? If so should we attribute this to a “real” effect or the result of measurement error?

Activity II 1. Repeat step 8 from Activity I but do so with the block flipped over so that now the opposite side of the block is in contact with the board. You’ll have to also repeat the data taking process with no extra weight added and with the 1 N weight added. Make a data table(s) below to list your data. Plot graphs as you did in Activity I. Test the validity of a linear model by finding the best fit line for each graph. Print these graphs (with appropriate labeling) and submit them with your report.

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Questions Does a linear model appear to describe the data in this case? Discuss. If we compare results from this activity with those of Activity I then we are investigating how the surfaces in contact (i.e. which surfaces) affect the friction forces. The best way to answer this questions is to look at how the graphs have been affected since they represent all of the data in each case. How has changing the surfaces in contact affected the graphs, and/or the best fit lines generated from them? Discuss. Activity III Another variable to consider is the surface area in contact. This variable can be studied by turning the block on its side and taking data. This data can then be compared to the previously collected data using the wooden side of the block. Here’s a way to make a comparison and see if the sliding friction is affected by surface area. Determine (just) the sliding friction force, with the block on its side, for 4 or 5 values of the normal force. Choose values of the normal force that are in the same range as the data you took before using the wood surface, but pick different values if possible. Then combine the data taking 4 or 5 values from each set and plot a graph of friction force versus normal force. Note that you are not plotting 4 columns of data just 2. The idea is to see if the two sets of data can be described by the same trend. Make a table below to list the new data. Fit a line to the data if that is possible and print the graph. Question(s) Does it seem reasonable that the two data sets can be described by the same trend? Discuss. What does this say about the affect surface area has on sliding friction? Discuss.