Factors of Sliding Friction Guide

Overview

Why is it harder to push a heavy object than a light one? Why is it easy to slip when walking on ice? This is 'kinetic friction' in physics at work. Through the classic 'control variable method', this experiment takes you to disassemble the three major variables affecting friction: normal force, roughness, and contact area. You will experiment personally to find out who is the real determining factor and who is the illusion that interferes with us.

Background

• 15th Century: Leonardo da Vinci was the first to systematically study friction, proposing the preliminary conjecture that friction is proportional to normal force and independent of contact area.
• 1699: French physicist Guillaume Amontons rediscovered the laws of friction and proposed the friction mechanism of interlocking rough surfaces.
• 1785: Charles-Augustin de Coulomb further distinguished between static and kinetic friction through experiments, perfecting the classical theory of friction.

Key Concepts

Formulas & Derivation

Experiment Steps

1. 1

   Explore Normal Force ($N$)

   Keep the board surface unchanged, add 1 hook code, then 2 hook codes to the block in turn, and click 'Pull at Constant Speed'. Observe the change in reading: the greater the pressure, the greater the friction.
2. 2

   Explore Roughness

   Remove the hook codes and switch the contact surface from 'Board' to 'Towel'. Since the towel is rougher ($\mu$ increases), you will find that even if the pressure has not changed, the friction has increased significantly.
3. 3

   Explore Contact Area ($S$)

   This is a key thinking trap. Keep the board surface and pressure unchanged, and change the block from 'Flat' to 'Side'. Observe the dynamometer: the reading actually remains unchanged. Conclusion: Kinetic friction is independent of the size of the contact area.

Learning Outcomes

• Master the two determining conditions of kinetic friction: normal force ($N$) and roughness of the contact surface ($\mu$).
• Learn to use the state of uniform rectilinear motion to measure the dynamic balance of forces.
• Clarify that friction is independent of contact area ($S$) and motion speed (within a certain range).
• Understand the core application method of the control variable method in physics experiments.

Real-world Applications

• Car Braking: Shorten the braking distance by increasing the normal force of the brake pad on the disc.
• Shoe Sole Patterns: Deepening the patterns is to increase the roughness ($\mu$) of the contact surface, thereby preventing slipping.
• Conveyor Belt: When transporting goods, prevent slipping by tightening the belt (increasing pressure) or increasing the roughness of the belt.

Common Misconceptions

Further Reading

• Physics Classroom: Friction