PHY 211 General Physics I Lab PCC Cascade page 1 of 6 Experiment Static and Kinetic Friction OBJECTIVES Use a Force Sensor to measure the force of static friction Determine the relationship between force of static friction and the weight of an object Measure the coefficients of static and kinetic friction for a particular block and track Use a Smart Pulley to independently determine the coefficient of kinetic friction for the same block and track Determine if the coefficient of kinetic friction depends on weight INTRODUCTION If you try to slide a heavy box resting on the floor you may find it difficult to get the box moving Static friction is the force that is acting against the box 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 The maximum static friction force is sometimes referred to as starting friction We model static friction Fstatic with the inequality Fstatic s N where s is the coefficient of static friction and N the normal force exerted by a surface on the object The normal force is defined as the perpendicular component of the force exerted by the surface In this case the normal force is equal to the weight of the object Once the box starts 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 kinetic friction In order to slide the box with a constant velocity a force equivalent to the force of kinetic friction must be applied Kinetic friction is sometimes referred to as sliding friction Both static and kinetic friction depend on the surfaces of the box and the floor and on how hard the box and floor are pressed together We model kinetic friction with Fkinetic kN where k is the coefficient of kinetic friction In this experiment you will use a Force Sensor to study static friction and kinetic friction on a wooden block A Smart Pulley will also be used to analyze the kinetic friction acting on a sliding block by measuring the acceleration MATERIALS Windows PC Photogate LabPro Interface Force Sensor Logger Pro Software block of wood with hook string gram scale Graphical Analysis Software mass set Smart Pulley PRELIMINARY QUESTIONS 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 PROCEDURE Part I Starting Friction 1 Measure the mass of the block and record it in the data table 2 Connect the Force Sensor to the Channel 1 input of the LabPro Interface 3 Open Exp 12a in the Physics with Computers experiments folder Set the range switch on the Force Sensor to 50 N One graph will appear on the screen The vertical axis will have force scaled from 0 to 20 Newtons The horizontal has time scaled from 0 to 5 seconds 4 Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block then place 1 kg on top of the block Be sure the mass is fastened so they cannot shift 5 Practice pulling the block and masses with the Force Sensor using this straight line motion Slowly and gently pull horizontally with a small force Very gradually taking one full second increase the force until the block starts to slide and then keep the block moving at a constant speed for another second 6 Sketch a graph of force vs time for the force you felt on your hand in step 5 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 7 Hold the Force Sensor in position ready to pull the block but with no tension in the at the top of the graph to set the Force Sensor to zero string Click on 8 Click to begin collecting data Pull the block as before taking care to increase the force gradually Repeat the process as needed until you have a graph that reflects the desired motion including pulling the block at constant speed once it begins moving Copy and Paste the graph into a Word document and print it for later reference 9 Choose Store Latest Run from the Data Menu to store the run as Run 1 for later analysis Part II Peak Static Friction and Kinetic Friction In this section you will measure the peak static friction force and the kinetic friction force as a function of the normal force on the block In each run you will pull the block as before but by changing the masses on the block you will vary the normal force on the block Mass Wooden block Pull Figure 1 1 Remove all masses from the block 2 Click to begin collecting data and pull as before to gather force vs time data 3 Examine the data by clicking the Statistics button The maximum value of the force occurs when the block started to slide Read this value of the maximum force of static friction from the floating box and record the number in your data table 4 Drag across the region of the graph corresponding to the block moving at constant velocity Click on the Statistics button again and read the average force during the time interval This force is the magnitude of the kinetic frictional force 5 Repeat Steps 2 4 two more measurements Record the values in the data table 6 Average the results to determine the reliability of your measurements 7 Add masses totaling 250 g to the block Repeat Steps 2 6 recording values in the data table 8 Repeat for additional masses of 500 750 and 1000 g Record values in your data table Part III Kinetic Friction Again In this section you will measure the coefficient of kinetic friction a second way and compare it to the measurement in Part II Using the Motion Detector you can measure the acceleration of the block as it slides to a stop This acceleration can be determined from the velocity vs time graph While sliding the only force acting on the block in the horizontal direction is that of friction From the mass of the block and its acceleration you can find the frictional force and finally the coefficient of kinetic friction Wooden block Push Figure 2 1 Connect the Motion Detector to DIG SONIC 1 of the …