Quick answer: Friction is the contact force that resists sliding. This AP Physics 1 lab on friction and inclines gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
A block is dragged over a board and later placed on a tilting ramp. By measuring the pulling force for different loads and the slipping angle, you find the coefficients of kinetic and static friction and test that friction is proportional to the normal force.
Level: AP Physics 1 (algebra-based) · Curriculum: AP P1 Unit 2
Aim
To find the coefficients of kinetic and static friction for a wooden block on a wooden board, and to show that friction is proportional to the normal force.
Equipment
- wooden block (0.40 kg) with a hook
- set of slotted masses (0.20 kg each)
- spring balance (0 to 10 N) and string
- wooden board about 1 m long, protractor or ruler
- balance
Safety
Keep feet away from the path of the block and masses, and hold the board firmly while tilting it so it cannot slip off the bench.
Method
- Weigh the block and add masses so the total mass m is 0.40, 0.60, 0.80, 1.00 and 1.20 kg.
- For each mass, pull the block along the level board at constant speed with the spring balance and read the force Fk in the middle of the run.
- Calculate N = mg for each mass (g = 9.81 N/kg).
- Tilt the board slowly with the 0.40 kg block alone on it, and measure the angle at which the block starts to slide. Repeat five times.
Results
Part 1: kinetic friction.
| m (kg) | N (N) | Fk (N) |
|---|---|---|
| 0.40 | 3.92 | 1.30 |
| 0.60 | 5.89 | 1.98 |
| 0.80 | 7.85 | 2.60 |
| 1.00 | 9.81 | 3.35 |
| 1.20 | 11.77 | 3.85 |
Part 2: angle at which the block starts to slide.
| Trial | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| θ (°) | 19.0 | 20.0 | 18.5 | 19.5 | 20.5 |
Analysis questions
Question 1 Normal force
Calculate the normal force and the friction ratio Fk ÷ N for the 1.00 kg row.Question 2 Graph of F against N
Calculate the gradient of Fk against N using the first and last data points and state its meaning.Question 3 Best value of kinetic friction
Calculate Fk ÷ N for all five rows and give the mean μk to two significant figures.Question 4 Angle method
Show that the block starts to slide when tan θ = μs, and calculate μs from the mean angle.Question 5 Uncertainty in μs
Use the smallest and largest angles to estimate the uncertainty in μs.Question 6 Comparing coefficients
Compare μs and μk and explain the difference.Question 7 Improvements
Explain why the block must move at constant speed, and suggest two improvements.
