Christina SvenssonMr WardleSPH3U- 0229.10.200802.11.2008

Determining coefficients of friction

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Purpose: Investigate the coefficient of kinetic friction for two materials and determine how mass affect the coefficient of friction.

Question: How does the mass of an object affect the coefficient of friction?

Hypothesis:For an increase in the normal force, there will be a corresponding decrease in the coefficient of friction.

Fg=m x g

Fg=Fn

Ff=M x Fn

∑ M= Ff / Fn

Design: This investigation of factors (in this case mass) affecting the coefficient of friction, we attached a certain mass to a spring scale (which measure force in Newtons) and pulled it across a table. The masses being used was 400 g, 700 g and 1000 g.

Independent variables: - Mass of object attached to the spring scale.

Dependent variables: - Uneven surface- Force of mass, N, shown on the spring scale.

Controlled variables: -

Materials: - Spring scale- 3 x 200 g mass- 1 x 100 g mass- 1 x 1000 g mass

Procedure: 1. Obtained the masses needed for the experiment2. Attached the 1st mass (2 x 200 g) to the spring and pulled it across the table.

Observe the spring and record data. Repeat procedure 3 times with the same mass.3. Same procedure was repeated with 700 g and 1000 g. Record data.4. Cleaned up lab area

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Observations: Refer to Table 1: Force of kinetic friction on selected masses. The data could be in error because the surface of the mass being used was slightly uneven, and that would lead to errors in calculations.

Analysis: Refer to Table 3: Coefficient of kinetic friction. As the results clearly shows, my results were wrong, and I have not proved my hypothesis. The smallest mass (400 g) had the smallest coefficient; 0.128 ± 10.83, while the largest mass (1000 g) had the greatest; 0.248 ± 2.89%. This lab did not validate my hypothesis. As mentioned in my observation; my results might have been in error because of the uneven surface of the mass used.

Evaluation:This lab was not a good lab. It was supposed to prove my hypothesis, which it did not do. My uncertainty for the 400 g mass is to large, and if I were to do it again, I would use a more accurate toll for measuring the force of friction. My errors were also big; I used cube-shaped masses, and they rolled around while pulling it over the table’s surface, causing the measuring of force of friction to constantly change (“needle” jumped up and down).

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Appendix

Table 1: Force of kinetic friction on selected massesSurface: Mass (g): Mass (N) Try 1 (N): Try 2 (N): Try 3 (N):

Table 400 4 0.4 0.5 0.5

Table 700 7 1.5 1.5 1.5

Table 1000 10 1.7 1.7 1.8

Table 2: Force of kinetic friction (average with uncertainties).Mass: 400 g 700 g 1000 g

Average of tries: 0.5 N ± 10.83% 1.5 N ± 3.33% 1.7 N ± 2.89

Uncertainty: Try 1) 0.05 / 0.4 = 0.125 = 12.5%

Try 2) 0.05 / 0.5= 0.1= 10%

Try 3) 0.05 / 0.5= 0.1= 10%

Try 1) 0.05 / 1.5~ 0.333~ 3.33%

Try 2) 0.05 / 1.5~ 0.333~ 3.33%

Try 3) 0.05 / 1.5~ 0.333~ 3.33%

Try 1) 0.05 / 1.7~ 0.294~ 2.94%

Try 2) 0.05 / 1.7~ 0.294~ 2.94%

Try 3) 0.05 / 1.8~ 0.0278~ 2.78%

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Table 3: Coefficient of kinetic frictionMass (m): Normal Force (Fn) Force of Friction

(Ff); data recorded of spring scale (average ± uncertainties).

Coefficient of friction ( μ )

400 g Fn = Fg

Fg = g x m = 9.8 m/s2 x 0.4 kg = 3.92 N

Ff = 0.5 N ± 10.83% = Ff / Fn

= 0.5 N / 3.92 N = 0.128 ± 10.83

700 g Fn = 9.8 m/s2 x 0.7 kg = 6.86 N

Ff = 1.5 N ± 3.33% = Ff / Fn

= 1.5 N / 9.8 N = 0.153 ± 3.33%

1000 g Fn = 9.8 m/s2 x 1.0 kg = 9.8 N

Ff = 1.7 N ± 2.89% = Ff / Fn

= 1.7 N / 6.86 N = 0.248 ± 2.89%

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