By: Rebecca Harvey
In this lab, we set up a sled with which we attached a force sensor. We would add different weights to the sled, and then pull the whole contraption and determine the force of that pull. Since the force of the pull is equal to the force of friction when the object is moving at a constant velocity, we could then measure the friction. We then flipped the sled, and used the side that had negligible friction for our second trial.good!
Setup:
Trial 1: (rubber - more friction)
Normal
Force (N)
|
Force
of Friction (N)
|
0.7
|
0.49
N
|
1.2
|
0.68
N
|
1.7
|
0.91
N
|
2.2
|
1.26
N
|
2.7
|
1.61
N
|
Analysis:
Verbal Model: As the normal force increases, the force of friction increases proportionally.
Math Model: Force of Friction = 0.56 (N/N)(Normal Force) + 0.03 N
Slope: For every 1 Newton of Normal Force, the Force of Friction increases by 0.56 N.
Y-Intercept: Close enough to 0 to be negligible. When there is no normal force, then the surfaces are not touching and there can be no friction between them.good
Trial 2: (Felt - less friction)
Normal Force (N)
|
Force of Friction (N)
|
0.7
|
0.05
|
1.2
|
0.12
|
1.7
|
0.18
|
2.2
|
0.27
|
2.7
|
0.35
|
Analysis:
Verbal model: As the normal force increases, the force of friction increases proportionally
Math Model: Force of Friction = 0.15 (N/N)(Normal Force) + 0.06 N
Slope: For every 1 Newton of normal force, the force of friction increases by 0.15 N.
Y-Intercept: Close enough to be negligible. When there is no movement, there is no friction. true but that's not why here, it was moving...
Conclusion:
Everyone in the class had the same y-intercepts on their graphs, 0, because when there is no normal force, the surfaces are not touching and therefore have no friction between them. The class data was very similar in that the slopes were all close to the same value, about 0.2 for the felt side of the sled. The rubber side of the sled had more variation in slope, from 0.5 to 0.7, because the rubber has more friction than the felt, which meant a larger slope in general, but because of this greater amount of friction, it was more difficult to pull the sled at a constant speed, therefore some groups had different data than other due to varying pulls.good
The slope is, physically, the friction relationship between two individual surfaces. This is known as the coefficient of friction (mu). This coefficient varies between different surface relationships, but is always the same between the two surfaces. For instance, concrete on copper is always the same as concrete on copper, but never the same as concrete on lead. excellentWe tested only two of these relationships: table vs. rubber and table vs. felt.
The slope is, physically, the friction relationship between two individual surfaces. This is known as the coefficient of friction (mu). This coefficient varies between different surface relationships, but is always the same between the two surfaces. For instance, concrete on copper is always the same as concrete on copper, but never the same as concrete on lead. excellentWe tested only two of these relationships: table vs. rubber and table vs. felt.
The way to calculate the force of friction is this equation: Ff = mu (FN)
The people wearing identical shoes could have different amounts of friction because they could have different masses and therefore the normal force of the ground pushing them up is different. This affects the force of friction by increasing or decreasing the normal force.yes
The people wearing different shoes could have the same amount of friction because in the equation, a smaller normal force with a greater coefficient of friction could balance out to be equal to a larger normal force with a smaller coefficient of friction. yesThe coefficients must be different because the shoes are different and the surfaces would therefore be different, but varying masses of the people could balance out the normal forces. Therefore, we can conclude that the force of friction is dependent on surface (coefficient of friction - relationship between surfaces) and the force of the surfaces pressed together (normal force).
Some sources of error include not pulling the sled at a constant speed, and irregularities in the table's surface (not being completely flat, having divots and scratches that increase friction). Next time, we could use a buggy to pull the sled, because it would move at a more constant rate than my hand.
Another factor that could affect the friction would be having the normal force pushing sideways rather than directly opposite form the force of gravity. This could be using a ramp instead of a flat table, and changing the angle of the slant of it. In the experiment, we could test how this affected both the direction and the force of friction. The controlled variables would have to be the same surfaces, the same mass of the sled, and a constant pull. This is really still testing the Fn, but changing the Fn in a different way (you will see when we do angles)
This lab worked, in that we understood mainly what to do and the whiteboarding process was quick and easy. I think the only thing I have to complain about is that we weren't sure what units to use. If that had been specified form the beginning it would have saved some trouble. Ok! :) Nice job.
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