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Microsoft Word - 3 30 2020 Friction LAB.docx PHYS 1401 SPRING 2020 Section 2103 PHYSICS 1401 SPRING 2020 Friction1 Equipment • Calculator Objective • To compare the coefficients of static and kinetic friction for two given surfaces. Introduction Friction is a resisting force that acts along the tangent to two surfaces in contact with each other when one body slides or attempts to slide across another. The direction of the frictional force is opposite the body's motion or attempted motion. Our simple model is that the available frictional force (??or ??) is directly proportional to the normal force (??). There are two different kinds of friction: the static friction force ??and the kinetic friction force ??. When two surfaces are still at rest, but attempt to slide one over another, static friction is present. The static frictional force ??is has a magnitude that can have any value, up to a limit: ??≤???? (1) Here, ??is the magnitude of the normal force between the surfaces. When two surfaces are moving with respect to each other, the other kind of friction occurs. It is the kinetic friction. The kinetic frictional force ??has a specific magnitude: ??=???? (2) In the expressions above ??is the static friction coefficient and ??is the kinetic friction coefficient. In general ??>??. This means that when enough force is exerted on an object to overcome the static frictional force and the object starts moving, the kinetic frictional force will be less than the applied force and the object will accelerate. Another way of thinking of this is that it is harder to get something moving than it is to keep it moving. 1 THIS LAB WAS TAKEN FROM THE PASCO EXPERIMENT MANUAL (7-FRICTION) PHYS 1401 SPRING 2020 Section 2103 Experiment 1: Static Friction Coefficient In this virtual experiment, you are asked to calculate the static friction coefficient of a block sitting on the lab bench. To do this, we will consider the limit of the static friction force by imagining that one is pulling very gently on the block until it just begin to move. Second, we can then calculate the normal force by assuming that the table surface is perfectly horizontal so that the normal force and gravitational forces on the block are equal in magnitude. One of the take-home lessons from this exercise, based upon the fact that as fs,max = µsn, the slope of the graph that denotes friction vs. normal force will generate the coefficient of static friction. Experimental Methodology 1. For this virtual experiment you will calculate the coefficient of static friction for 2 blocks of different mass, whose bottoms has been “roughed up” with sandpaper. Block1 has a mass of 200 grams and block2 has a mass of 300 grams. The rough side of the block is in contact with the cable. 2. Imagine that you have tied a force sensor to your block as shown in Figure 1. Figure 1. Schematic setup for experiment 1. 3. You place the blocks on the table. You conduct five trials and find the average value of the force on each block. As you conduct your experiment, you gradually increase the force on each block and observe that the block does not move until you reach a specific value of the force in. which the block “just” begin to move. You find that that for block 1 the average force of your observations is 0.98 N + 0.15 N. You repeat the same procedure for block 2, making note that the average force that “just” causes the block to move is 2.21N + 0.25 N. 4. For each block fill in the data below and perform the Data Analysis as noted below. Mass of Each block(kg) Normal force (N) Range of Calculated Values for ?? Range of Calculated Values for ?? (N) Block 1 Block 2 PHYS 1401 SPRING 2020 Section 2103 Data Analysis 5. Show your calculations to determine the following: • Calculate the normal force ??assuming it equals the gravitational force on the total mass. • Calculate the range of the static frictional force based upon maximum, minimum and mean values. • Calculate the range of the coefficient of static friction based upon the maximum, minimum and mean values of the frictional force values. 6. Conduct an internet search to see if you can identify the most likely material for each block (i.e., wood, glass, steel) based upon your calculated range of values. See reference below for a starting point in your search. Experiment 2: Kinetic Friction Coefficient Measuring the kinetic friction coefficient is trickier because we can only measure the pulling force, not the friction force directly. They are only equal if the block is moving with zero acceleration. As you are aware, it is difficult for a person to pull on the force sensor with a zero acceleration. However, we can construct a virtual experiment approximately emulating constant acceleration. A block of mass ?block(usethetwoblocksinexperiment1)is placed on the table-top, and a weight hanger of mass ?hangis tied to a string, which is passed over a pulley (massless, frictionless) paired with a photogate and tied to the block, see Figure 2. When the weight hanger is released, the entire system will accelerate and measure that acceleration. Figure 2. Schematic setup for Experiment 2 The external forces affecting the acceleration of the system are: • ??pulling down on the mass hanger. • ??resisting the motion PHYS 1401 SPRING 2020 Section 2103 Since these forces are in opposite directions, the sum of the forces is the difference of their magnitudes. This must equal the total mass times the acceleration. ?net,ext=?hang?−??=?total? (3) The vertical forces on the block are gravity and the normal force and assuming a perfectly horizontal table-top, they are equal and opposite. ??=?block? (4) So here’s the basic idea: Find ??for each of the blocks. Virtually, set up each block as shown in Figure 2 and place 450 grams on the hanger for block 1 and 550 grams on the hanger for block 2. Now, imagine that you release each block and the system approximates uniform motion. Your virtual Smart Meter records the average acceleration values for block1 of 6.03 m/s2 + 0.75 m/s2 and average acceleration values of block2 of 5.1 m/s2 + 0.45 m/s2 . Experimental Setup and Data Recording Mass of the block (kg) Hanger mass (kg) Normal Force (N) Average Frictional Force (N) Tension in Each Cord (N) Measured Acceleration of System (m/s2) Data Analysis 1. Calculate ?? by solving Equation (3), and calculate ??using the mass of the block. • Solve ??=????,andthendeterminetheaveragevaluesofTensionforeachsystem. Final Comparison Organize the results in the summary table above. Based upon your investigations of block materials you found in Experiment 1, do the coefficients of kinetic friction seem to match the expected values of each material? https://www.engineeringtoolbox.com/friction-coefficients-d_778.html