Friction - Lab 5

Friction - Lab 5: Introduction Friction is a force parallel to the surfaces when two solids, for example, come in contact with each other and prevents motion between the two surfaces. Friction depends on the materials of the two surfaces and the normal force between the two surfaces. Coefficient of friction is a value showing the relationship between the force of friction of two objects and their normal force. Static friction is what oppose and keeps the surfaces of those two objects from slippage. It’s value is the same as the applied force but negative and opposite in direction so if the applied force changes, so will static friction. Coefficient of static friction is a number that depends on the material composition and roughness of both surfaces. Kinetic friction is after slipping occurs between two surfaces and opposes friction force just to keep the movement going. Kinetic friction has the same constant value anytime the surfaces move with each other, unless those surfaces change. Coefficient of kinetic friction is a number less than the coefficient of static friction since frictional force decreases after moving. To understand forces and types of friction, we run an experiment to test an object on each part of the friction board. This will demonstrate static and kinetic friction between the object and different materials, while also collecting data that will be used to reach the coefficients’ values. Different test runs are part of this experiment to determine whether coefficient of friction depend on varying surface areas, weight and if speed alone determines kinetic coefficient of friction. Experimental Equipment used: ● PASCO Economy Force Sensor with hook attachment ● A length of string ● A Friction board split into 4 sections (Ours appeared to be a Painted Vinyl, Sandpaper[between 400 and 600 grit. Though exact grit unknown], Corkboard, Plywood[the one used in this experiment appeared slightly weathered]) ● A wood block with a hook ● 2 200g weights In order to confirm the correlation between force and the coefficient of friction under different conditions we had to perform multiple experiments. First to determine the coefficients of friction, we used the PASCO program and the Force Sensor to pull the wood block horizontally across each of the 4 sections. By carefully observing the graphs we could determine the FMax by the highest point on the graph and use a line of best fit to determine the FSteady  for each of the materials. Using the FMax  we could find the μs (static coefficient of friction) and FSteady to find μk(Kinetic coefficient of friction). For the purpose of consistency for all parts following this we used the plywood material. In the next section we wished to compare how the surface area affects the μ.Just as in the previous section we dragged the wood block with the sensor horizontally across the material, first with the larger surface area as we did in the first section and once more along the skinny side of the wood. We performed this particular section about three times in order to get consistent data. The third experiment wished to explore how mass affects the μ. The set up was identical to the first section starting with finding the μs and μk of the block by itself being pulled across the plywood horizontally. For each subsequent trial we added a 200g weight to the wood and repeated the process. The final experiment explored how speed affects the μ. We simply pulled the wood block and sensor at three different but consistent speeds across one material to find the μk. Data/Analysis Predictions: Smallest μ Largest μ Cork board plywood Painted wood sandpaper Q. The force versus time graph has a typical shape like seen in the illustration above. In the three spaces below, explain what is happening to generate this distinctive graph. 1. The pulling force f(t) starts at zero and increases because… The force increases because it has not yet reached the same force as the static friction. The force will continue to rise until the coefficient changes to kinetic, which causes a smaller force. 2. The pulling force f(t) reaches a maximum and then decreases because… the coefficient of static friction is larger than the kinetic coefficient. The surface provides a higher friction force before it moves than when it is in motion. When the force reaches the point where it is equal to the maximum force of static friction, the object will begin to move. Now the coefficient of friction is kinetic, which is lower and requires a smaller force to continue to move the object. 3. The pulling force f(t) attains a steady value because… The force required to keep moving the object has been reached. The constant force is applied, creating a net force of 0 between the object and the surface. Experiment 1: Normal force: 1.02 N Friction Board Material fmax (N)  fsteady (N) μs μk Cork board 0.200 0.085 0.196 0.083 plywood 0.633 0.312 0.621 0.306 Painted wood 0.800 0.499 0.784 0.489 sandpaper 0.400 0.241 0.392 0.236 Results for all 4 friction board materials in graph form are given below Graph of Cork Board This graph was the result of pulling the wooden block horizontally on the cork part of the friction board. For every part of the board, after we start timing, a member of the team will zero the force sensor which is represented from the increase in the force line in time interval 1.4 to 1.8 seconds. The force values at 0 to 1.4 seconds are not included in our search for the fmax , fsteady or μk because the wooden block is not moving and the data coming after zeroing the sensor is the result of force required to maintain a constant motion for kinetic coefficient. The data values increasing and decreasing rapidly in time interval 1.8 to 3 seconds represents the force steadying out, it’s also boxed and, with the linear feature in Capstone, gave the most steadiest line in green and the y-intercept that is the value for fsteady. Also, the highest force value (fmax  ) was found to be within the time interval 1.4 to 1.8 seconds. Graph of Plywood Data from 0 to 1.8 seconds weren’t included in our search for fmax , fsteady or μk, for the same reasons from the previous graph. After zeroing the force sensor, the force increases and the maximum point of that increase is our fmax  . The highest force is greater than the cork’s highest value because there's more friction. The lines boxed represents the force steadying out from 2.4 to 4.1 seconds. The y-intercept from the linear blue line, which is intersecting with the highlighted boxed points, represents the steadying force. This graph shows how there wasn’t a steep drop like the cork graph but the lines aren’t increasing and decreasing rapidly due to the fact that after so many practice rounds, the team was able to maintain a constant force when pulling with the force sensor. Graph of Painted Wood The highest maximum force value is found in time interval 1.2 to 1.4 seconds. The lines showing force is increasing and decreasing more consistently and near the horizontal line of .5 Newtons. Because of this, the steadying force is very close to .5 N and the kinetic coefficient is higher than the rest due to the painted wood having the most friction and maintaining a constant force while pulling. Graph of Sandpaper This graph holds a longer time but not the highest force value. Also, zeroing the force sensor isn’t properly shown and probably not done correctly. However the highest force value was collected and the steady force value was still found due to the fact that the data was decreasing and increasing close to .245 N. The steady force value is found in time interval .9 to 4.6 seconds, the longest time frame out of all the other parts in the friction board but gave a straight line for the steady force value from the linear equation. Q. Do your data show the ranking you expected based on the qualitative observation results? No. Our data proved that the cork board was ranked correctly, but the sandpaper should have been placed before the plywood to have been correct. Q.Do your data indicate that the static coefficient of friction is generally larger than the kinetic coefficient for a given pair of surfaces? Justify your answer. Yes. For each run we performed on different surfaces, the static coefficient of friction was always safely above the value of the kinetic coefficient of friction. Experiment 2: Plywood Amount of slider area in contact with board fmax (N)  fsteady (N) μs μk Small area 0.565 0.300 0.554 0.294 Large area 0.633 0.312 0.621 0.306 Experiment 3: Normal force: 2.98 N, 4.94 N Mass added to blocks fmax (N)  fsteady (N) μs μk 0 kg 0.633 0.312 0.621 0.306 0.2 kg 1.354 0.795 0.454 0.267 0.4 kg 2.317 1.400 0.469 0.283 Graph after 0.2kg added The force required to move the heavier wooden block has increased so the maximum force increases as well. But there is little difference between static and kinetic coefficient when .2 kg and .4 kg is added. Graph after 0.4kg added Data collected after 2.6 seconds wasn’t collect because we did not stop recording, accidently. Q. Does your data indicate that the coefficients of friction depend on the total mass (block and added masses) of the objects? If so, is the difference significant? Compare the relative % difference to measurements (small versus large mass). The data indicates that the coefficients of friction do not depend on mass. Although the force required to move the object increased, it was the normal force that increased with mass, meaning the coefficient of friction did not change. Experiment 4: Normal force: 1.02N fsteady (N)  μk Slow speed 0.304 0.298 Moderate speed 0.303 0.297 Fast speed 0.396 0.388 Does your data indicate that the coefficient of kinetic friction depends on the speed of the two surfaces touching? If so, the difference is remarkable? Compare the relative % difference to measurements (small versus large speed). The data indicates that the coefficient of kinetic friction does not depend on speed. The change in speed did not even change the magnitude of the force. Some of the error in the fast speed test can be attributed to the force sensor not always zeroing correctly. The final speed test was also not as accurate since we were running low on time to finish the lab. slow Low speed had allowed more time to accurately collect a longer time frame. This time frame gave us a steady force that was close to the other graphs with moderate and fast speeds. moderate fast Due to the short time, you can see how force increases and decreases rapidly. However the steady force is still close to the previous graphs. Conclusion After our experiment, we were able to observe the static and kinetic friction between different materials by pulling on a wooden block over different parts of the friction board several times. We had seen that the coefficient of kinetic friction does not depend on speed and doesn’t change the magnitude since it stays the same after being pulled in various speeds. Coefficients of friction doesn’t depend on surface area or weight because, despite the increase in applied force, the coefficient of friction itself didn’t change.