Quick answer: Newton's second law states that the resultant force equals mass times acceleration, ΣF = ma. This Physics 11-12 lab on friction and inclined planes gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How rough is a surface? In this lab you tilt a board until a wooden block just starts to slip, and use the angle to calculate the coefficient of static friction without any force meter. You then repeat the test with sandpaper to compare two surfaces.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB A.2
Aim
To measure the coefficient of static friction between a wooden block and a board from the angle at which the block starts to slide, and to compare two surfaces.
Equipment
- Smooth wooden board about 0.60 m long, wooden block
- Strip of sandpaper that can be taped to the board
- Books or a clamp stand to raise the board, metre rule
- Protractor (optional)
Safety
Keep fingers clear of the block and the gap between the board and the table when raising the board. Place a soft towel at the bottom so that the block cannot fall on a foot.
Method
- Place the block at the top part of the flat board and raise one end very slowly.
- Stop at the moment the block just starts to slide, and measure the height h of the raised end above the table. The board length is L = 0.60 m.
- Repeat five times, putting the block back each time.
- Tape the sandpaper to the board and repeat the five measurements.
- Calculate sin θ = h ÷ L, then θ and μs = tan θ for each surface.
Results
Sample measurements of the height h at the moment of slipping:
| Trial | Wood on wood: h (m) | Wood on sandpaper: h (m) |
|---|---|---|
| 1 | 0.27 | 0.39 |
| 2 | 0.28 | 0.41 |
| 3 | 0.26 | 0.40 |
| 4 | 0.29 | 0.40 |
| 5 | 0.27 | 0.40 |
Analysis questions
Question 1 Mean heights
Calculate the mean height h for each surface and the absolute uncertainty for the wood-on-wood surface using half the range.Question 2 Angles
Calculate sin θ and θ for each surface.Question 3 Proving the method
Show that μs = tan θ at the point of slipping.Question 4 Coefficients
Calculate μs for each surface.Question 5 Uncertainty
Estimate the uncertainty in μs for the wood surface from the smallest and largest heights.Question 6 Sliding down
Once moving, the wood-on-wood kinetic coefficient is 0.40. Calculate the acceleration of the block on a 30° slope.Question 7 Effect of mass
Explain why putting a second block on top does not change the slipping angle.Question 8 Improvement
Suggest one improvement to reduce the uncertainty.
