Physics Investigatory Project (Class XI)
Title: Study of Friction: Determination of Coefficients of Static and Kinetic Friction and Their
Dependence on Normal Reaction and Apparent Contact Area
Abstract
Friction is a resistive force that opposes relative motion between surfaces in contact. This project
determines the coefficients of static friction (µs) and kinetic friction (µk)...
Objectives
1. Measure µs between a block and different surfaces.
2. Measure µk using uniform–speed motion.
3. Verify limiting friction proportional to normal reaction.
4. Investigate dependence on contact area.
Theory
Static friction acts up to limiting friction (F■ = µs N). Kinetic friction is Fk = µk N with µk < µs.
Friction is largely independent of contact area.
Apparatus & Materials
Wooden block with hook, surfaces (glass, cardboard, sandpaper), spring balance, slotted masses,
pulley, inclined plane with protractor, scale, stopwatch.
Procedure
A. Static Friction: Pull block with spring balance, note maximum force before motion.
B. Kinetic Friction: Pull at constant velocity, record steady force.
C. Inclined Plane: Measure critical angle θc where block slides.
D. Contact Area: Compare results with different block faces.
Sample Calculations
From slope of F■ vs N graph, µs = ∆F■/∆N. From Fk vs N, µk = ∆Fk/∆N. For inclined plane, µs =
tan θc.
Results
µs ≈ 0.40, µk ≈ 0.30 (for cardboard). µk < µs verified. Friction independent of contact area.
Sources of Error
1. Parallax error.
2. Uneven pulling.
3. Dust or wear on surfaces.
4. Calibration drift in spring balance.
Precautions
1. Surfaces clean and dry.
2. Pull slowly and steadily.
3. Center added masses.
4. Repeat trials and take means.
Applications
Used in brakes, clutches, tyre-road grip, robotics grippers, flooring safety.
Conclusion
Results support F ∝ N, µk < µs, and friction independent of contact area. Experimental µ values
match expected ranges.
Sample Observation Table (Static Friction)
Added Mass (kg)
Normal Load N
Trial 1 (N)
Trial 2 (N)
Trial 3 (N)
Mean F■ (N)
0.00
9.8
3.1
3.0
3.2
3.1
0.10
10.8
3.5
3.5
3.6
3.5
0.20
11.8
3.9
4.0
3.9
3.9
0.30
12.8
4.3
4.2
4.3
4.3