Modeling Friction Forces
Annemarie Branks
Professor Wolf
Part 1:
Objective: Determine the coefficient of static friction between two surfaces by graphing the normal forces and frictional forces for an increasing number of masses.
Procedure:
1. With your block on the table, and the string tied to the Styrofoam cup hanging over the edge via "Atwood" pulley, begin slowly filling your Styrofoam cup with water until you just barely see the block begin to move.
2. Measure the mass of the cup and the water on a scale. In kilograms, multiply the mass by the force of gravity. This number will be equivalent to the max static friction force of the block.
3. Repeat steps 1 and 2 with an additional block for a total of four trials. You will also need to weigh the mass of the blocks in each trial to find their Normal Forces.
4. From this information we can create a Max Static Friction Force by Normal Force graph and find a best of fit equation. The slope of the graph will give us the coefficient of static friction for the block against the table. The line on the graph must intercept the origin.
Our μ for static friction is 0.3267.
Part 2:
Objective: Determine the coefficient of kinetic friction between two surfaces by graphing the normal forces and frictional forces for an increasing number of masses.
Procedure:
1. Tie a force sensor to your first wooden block that will remain in contact with the table through out the entire experiment. After you've set up the force sensor with LoggerPro and zeroed it, you can begin collecting data by pulling on the force sensor horizontal to the table at a constant speed.
2. Obtain the average force exerted on the block from LoggerPro.
3. Record your mass of the block so you can find its Normal Force.
4. Repeat steps 2 and 3 for a total of four trials, with each trial having an additional mass.
5. Just like in Part One, make a graph in excel to find the coefficient of friction, but only this time it will be for kinetic friction. It would make sense that the coefficient for kinetic friction is smaller than the coefficient for static friction.
Our μ for kinetic friction is 0.2675.
Part 3:
Objective: Find the angle at which the block begins to slip to determine the coefficient of static friction between the block and the surface.
Procedure:
1. Place your block on a track that you can move up and down. Have a lab partner hold down one end of the track so that it does not move and affect the results.
2. Slowly raise one end of the track and watch for when the block begins to slide down. At the moment, record the angle of the track. Our angle was 13.5° +/- 0.1°.
3. You may want to draw out a diagram displaying all the forces acting upon the block. Solve for μ for kinetic friction. In our case it was 0.2401.
Part 4:
Objective: Determine the coefficient of kinetic friction for an object sliding down a slope.
Procedure:
1. Set up a track to rest on a stand at an angle large enough to cause the block resting on it to move. Record that angle.
2. Keeping the track at that angle, set up the motion detector at the top end of the track so you can determine the block's acceleration.
3. With the determined values for the block's acceleration, the track's incline, and the mass of the block, we can solve for μ for kinetic friction. For our experiment, LoggerPro gave us an acceleration of 1.079 m/s^2, the track was at an incline of 24.3°, and the mass of the block was 0.1264 kg.
From the sum of forces in the y direction, we found that the Normal = 1.13N. Using Normal*μ to replace Frictional force, we solved for μ and got a value of 0.33.
Part 5:
Objective: With our new found coefficient for kinetic friction from Part 4, calculate an expression for what the system's acceleration should be if the track were laid horizontally.
Procedure:
1. Create a system where the block is still on the track and will accelerate due to a hanging mass. You will use LoggerPro to find the actual acceleration of the system so you can compare it to the your calculated acceleration.
2. Before you start the physical experiment, find your calculated acceleration. Here is our calculation for acceleration.
3. We can now collect our data via LoggerPro when we let go of the hanging mass and find acceleration. Our acceleration was 0.6911 m/s^2. This means our percentage difference was 40.15% which seems large but Professor Wolf said the percentage difference should be off by about 50%. This could be because of inconsistency in our track and string tension as well as not considering the pulley as a part of the system.
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