LAB 05B: Friction 2 times PURPOSE BACKGROUND MATERIALS PRELIMINARY QUESTIONS: (TO DO BEFORE THE LAB!!) Lab 05B: Friction 1/5 Mr.

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1 LAB 05B: Friction 2 times PURPOSE To investigate how friction is related to other variable such as the normal force, weight, and surface coefficients. In this experiment we will determine the static and sliding coefficients of friction in a traditional way and a non traditional way. BACKGROUND If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving because friction forces act between the object and the surface. STATIC FRICTION inhibits the object from moving from rest. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. Once the object begins to slide you must continue to exert a force to keep the object moving, or friction will slow it to a stop. The friction acting on the box while it is moving is called SLIDING (OR KINETIC) FRICTION. In order to slide the box with a constant velocity (F=0), a force equivalent to the force of kinetic friction must be applied. There is also a material property we will just call the coefficient of friction for now. One of your goals is to determine what this thing is and how it is related to friction. At this point, all we know is that it is a material property, and will will give it the name (pronounced mu, like a French cow). MATERIALS Windows PC with LoggerPro Software and Vernier Dual Force Range Meter OR Spring scale(s) (with capacity to measure weight of shoes) Microsoft Excel/LoggerPro Data Analysis or similar program Block to drag Tilting surface with angle measurements to drag on Connecting Material: String (1-2 m) / Tape Various masses PRELIMINARY QUESTIONS: (TO DO BEFORE THE LAB!!) (PLEASE ANSWER IN YOUR LAB NOTEBOOK) 1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your choice? 2. How do you think the force of friction is related to the weight of the box? Explain. 3. Do you think the material you are sliding something across has any effect on the frictional force, and if so what effect? 4. Draw a Free-Body Diagram of a block that is being dragged along a horizontal surface at a constant velocity. 5. Draw a free body diagram for a block resting on a inclined hill just before is starts to slide. 6. Sketch what you think the shape of a Force vs. time graph would look like for dragging an object from rest along the ground. Lab 05B: Friction 1/5

2 PROCEDURE THE VARIABLES: First we have to figure out what friction depends on. In our class discussion, we came up with a few ideas: the normal force and the surfaces. If we work on flat ground with no upward external forces, we can assume the normal force is equal to the object s weight, which is pretty easy to control. We can also switch surfaces between wood and the table top. We do not have any numbers for the surfaces, but we can at least see if changing the surface has any effect, and maybe create some numbers to go with them. Remember we can only vary one of these at a time. This lab will be a little different. At certain points I want you to stop so we can discuss quickly as a class. When it is noted, please pause. You will be asked questions as you go through the activity. Record your answers in your lab notebook. 1. Start by setting up the block-basket on the wood hinged thing and attach the force sensor to it. Add one 500 g mass or two plates depending on your materials. 2. If you slowly pull on the force sensor what do you feel? You should feel the force you are applying increase until eventually the block starts to slide. 3. Start collecting data and gently pull the block to the side at a constant velocity. You should be able to fit 2 or 3 trials on one graph. Q2: What does the graph look like? Can you determine at which point it started to slip? Can you determine where it was sliding with a constant velocity? Defend your answers. 4. Record the values of the maximum static friction and the average kinetic friction from your graph. You should do 3-4 trials so you can take an average. Make sure to save one of these graphs the easiest way is to copy them and paste them into a Word document that you can to yourself. 5. Part A: VARY THE NORMAL FORCE Repeat the process with different normal forces. Add more mass to your cart to increase the normal force and take 3-4 readings of the static and kinetic friction force. Make sure to record all the masses being added. Repeat until you have 4 different values. You do not need to save these graphs. 6. Part B: VARY THE SURFACE Place the cart on the table top and drag it across with the force sensor. Take readings for the same normal force values you previously used in step 5. Q3: What do you think the friction between two surfaces depends on? How is it related to the normal force? What role does the coefficient play? 7. Part C: A SECOND WAY (if you have time and want to impress me) There is another way to determine the friction between two surfaces, and ultimately the coefficient of friction between two surfaces. This can be done by placing your cart on a platform and then gradually incline the platform. At a certain angle, the cart will start to slip. 8. Place the cart on the incline and gradually incline it until the block starts to slip and record this angle. Repeat this 3 more times so you can average them. Repeat this process for two of the other values for the cart weight that you used in step 5, making sure to record all the angles. 9. From your free body diagram of a block on a hill and your knowledge of the force of friction, see if you can determine a relationship between the angle of the hill and the coefficient of friction. Lab 05B: Friction 2/5

3 ANALYSIS 1. Using your Free Body Diagram and observations, create an equation for the coefficient of friction of your cart in terms of its weight F g and the force of friction F f. 2. Look at one of the graphs you saved in the first part. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed. Compare the force necessary to keep the block sliding compared to the force necessary to start the slide. 3. Using the Equation F f = F N calculate the coefficient of friction for each Normal Force based on your average value for the friction force and record it in the Data Table. 4. Create a graph of your pulling force vs. normal force for your data. Find the slope of this graph. What does it represent? What units is the slow measured in? 5. Using Microsoft EXCEL or LoggerPro, create a Scatter-Plot of the Average Force of Friction vs. Normal Force. Normal Force should be on the x-axis, and Friction Force on the y-axis. Create one for the Static Friction data and one for the Kinetic Friction data for each of the two surfaces combinations. (you may put them on the same graph, but make sure they are labeled). 6. Using the TRENDLINE function in EXCEL, Add Trendline and check Show the Trendline Equation on your plot. Since F f = * F N, the slope of this line should be the coefficient of friction of the cart-surface combination. Record values for the static and kinetic coefficient of friction for each surface combination. 7. Explain any differences you see between the static and kinetic coefficients of friction for an individual object. Which one is greater? Why? 8. Compare the different ways of finding the friction coefficient with each other by taking % differences and record them in the data table. (%diff = [difference / average] x 100%) 9. Compare the calculated coefficients of friction with the values for the friction force. Does a bigger friction force mean a bigger coefficient of friction? Why or why not might this be the case? 10. Using your diagram and data for the Second Way, determine the static coefficient of friction between the cart and the wood. What equation did you use? Compare this value to the value determine in the classical way? How close are they? Why might they not be exactly the same? 11. Athletic shoes tend to have a greater coefficient of friction than dress shoes. Why might they be designed that way? What part of their design may attribute to their higher coefficient of friction? 12. Rubber is often placed on stairs and sand is often placed on ice to make them safer. What would this do to the coefficient of friction? Why does this make them safer? Lab 05B: Friction 3/5

4 OBSERVATIONS AND DATA (TO TURN IN) Group Members: Mass of Empty Cart: Part A: Classical Investigation of Friction on Normal Force Static Friction s Kinetic Friction k s from graph: K from graph: Part B: Classical Investigation of Friction on Table Surface Normal Force Static Friction s Kinetic Friction k s from graph: K from graph: Lab 05B: Friction 4/5

5 Part C: Determination of sliding angle on wood. Cart Weight Angle () Coefficient of friction s Average Summary of Calculated Coefficients of Friction s and k for sections A - C Surfaces By Averages By Graph Table Surface By Averages Table Surface By Graph By Angle s k % Difference of S between Average and Graph Technique for Wood: % Difference of S between Average and Graph Technique for Table: % Difference of K between Average and Graph Technique for Wood: % Difference of K between Average and Graph Technique for Table: % Difference between Drag and Angle Technique for Wood: Lab 05B: Friction 5/5