The method and the results are on the friction and inclines lab. Do the analysis yourself first, then check the answers below.
Quick answer: Friction is the contact force that resists sliding. These answers go with the AP Physics 1 lab on friction and inclines: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The model answers use the sample data. Your readings will differ slightly, but μk should come out near 0.3 and μs a little larger than μk for wood on wood.
Level: AP Physics 1 (algebra-based) · Curriculum: AP P1 Unit 2
N = 1.00 × 9.81 = 9.81 N. Fk ÷ N = 3.35 ÷ 9.81 = 0.34.
Question 2 Graph of F against N
Gradient = (3.85 − 1.30) ÷ (11.77 − 3.92) = 2.55 ÷ 7.85 = 0.32. It is the coefficient of kinetic friction, since Fk = μkN.
Question 3 Best value of kinetic friction
Fk ÷ N = 0.332, 0.336, 0.331, 0.341, 0.327; the mean is 0.33.
Question 4 Angle method
On the point of slipping, mg sin θ = μs mg cos θ, so tan θ = μs. The mean angle is (19.0 + 20.0 + 18.5 + 19.5 + 20.5) ÷ 5 = 19.5°, so μs = tan 19.5° = 0.35.
Question 5 Uncertainty in μs
tan 18.5° = 0.335 and tan 20.5° = 0.374. Half the spread is (0.374 − 0.335) ÷ 2 = 0.02, so μs = 0.35 ± 0.02.
Question 6 Comparing coefficients
μs = 0.35 is slightly greater than μk = 0.33. The surface bonds that must be broken to start the motion are stronger than those continually reformed while sliding, so μs is greater than μk.
Question 7 Improvements
At constant speed the net force is zero, so the balance reading equals the friction force. Improve the lab by dragging at a slow steady speed with a force sensor, and by measuring the mass of the block and the masses with the same balance. The balance reading uncertainty is about ±0.1 N.
