22-March-2017: Modeling Friction Forces

Lab#7: Modeling Friction Forces

May Soe Moe

Lab Partners: Ben Chen, Steven Castro

22-march-2017

Objective: To produce a model that gives the coefficient of static and kinetic frictions through five different experiments

Introduction: We had learned that there are static and kinetic friction forces acting on an object if the surface is not totally smooth. Static friction is the friction force between two bodies when they are in fixed position. Kinetic friction is the friction force between two bodies when they move and slide against each other. We also have equations for both friction forces, which are f(static maximum)= μs*N, and f(kinetic)=μk*N. μs indicates the coefficient of static friction whereas μk indicates the coefficient of kinetic friction, and N indicates the normal force of an object. We used these equations and set up scenarios where we could figure out the coefficients of static and kinetic frictions by manipulating these equations, calculating them mathematically, or by figuring out through the graphs.

Experimental Procedures:

Experiment 1: Static Friction

(1) Set up the lab as above. We used a white board and a block with linoleum tile attached to its bottom due to their smooth surface. We also connected the block with a pulley to have another block hanging vertically as in the picture. Through this experiment, we will find out the tension, which was also the maximum static friction force. How we figured out the tension or the maximum static friction force was by adding masses to the hanging mass until the block on the white board started to move. We taped the initial position of the block--using blue tape on the white board--to let us know if the block has moved or not. When the block started to move, we removed the latest mass that made the block move and recorded the hanging mass. The mass that made the block move was 70 g.

(2) We kept repeating the same procedure by putting 200 g on top of the block during second scenario, 400 g in third scenario, 600 g in fourth scenario and 800 g in fifth scenario, and kept adding the masses to the hanging mass until the block started to move. We recorded them and the masses came out to be as below:

In this graph, the y-axis represents the static friction of the hanging mass; the x-axis represents the weight of the block or Normal force; and the slope represents the coefficient of static friction or mg.

Therefore, from this graph, the coefficient of static friction is 0.4125.

Experiment 2: Kinetic Friction

(1) We figured out the coefficient of the kinetic friction through this experiment. 

(2) Frist, we tied and connected a block and a force sensor with a string, connected to the computer with LabPro. We calibrated the force sensor using 500 gram hanging mass so that we could get reliable data. 

(3) After the calibration, we slowly pulled the board instead of a block at a constant speed and collected data with LabPro. We averaged the force exerted on the board using Statistics from Analyze menu and recorded the average value. Constant speed in this case made the net force equaled to zero. From the graph, we got the coefficient of kinetic friction to be 0.5197, which is the y-intercept of the graph. 

Experiment 3: Static Friction from a Sloped Surface

(1) We placed a block horizontally on the white board and raised one end of the board, tilting it until it started to slide down. The angle when the block started sliding down was 24Degree, and we used this angle to determine the coefficient of static friction between the block and the surface. 

Experiment 4: Kinetic Friction From Sliding A Block Down An Incline

(1) Putting a motion detector at the top of an incline which made the block accelerate down the incline. We measured the angle of the incline that made the block slide down and recorded the acceleration of the block using the LoggerPro and motion detector.

 (2) From the graph, acceleration of the block is the slope of the velocity versus time graph, which is 1.912m/s^2. Through calculation using Newton's Second Law and equation of kinetic friction, the coefficient of kinetic friction came out to be 0.312.

Experiment 5: Predicting the Acceleration Of a Two-Mass System

(1) Using the coefficient of kinetic friction I got from experiment 4,we derived an expression for what the acceleration of the block would be if we used a hanging mass sufficiently heavy to accelerate the system. 

Comparing our acceleration from velocity versus time graph and acceleration from derivation and experiment, our mathematical acceleration was off by 0.568. 

Conclusion: This experiment had a lot of uncertainties and many rooms for errors. This experiment cannot be done repeatably because you will not get the same results from different trials.