# 8: Frictional Forces

{'English - US': '/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-8/lec8.srt'}

Flash and JavaScript are required for this feature.

Download the video from iTunes U or the Internet Archive.

Topics covered: This lecture deals exclusively with frictional forces.

Instructor/speaker: Prof. Walter Lewin

Date recorded: September 24, 1999

## Video Index

Please make sure you play the Video before clicking the links below.

• Normal and Frictional Forces
Starting with a block at rest on a horizontal surface, Professor Lewin describes the normal force, the maximum frictional force that must be overcome to budge the block, and the coefficient of static friction. Once the block is budged and begins to slide, you encounter a smaller coefficient of kinetic friction.

• Measurements of the Coefficient of Static Friction
Consider a block at rest on an inclined plane, and increase the tilt until the block just starts to slide. By measuring this tilt angle you can calculate the coefficient of static friction. The friction coefficient only depends on the materials in contact; it is independent of the mass of a sliding object and independent of its surface area (this is rather non-intuitive).

• Another Way to Measure Friction
The system consists of a block on an inclined plane. The block is counterbalanced by a second mass connected to the block with a massless string and a near massless, frictionless, pulley. The direction of the frictional force depends on whether the block on the incline wants to move uphill or downhill or stay at rest. By increasing the second mass until the block on the incline budges uphill, you can measure the coefficient of static friction.

• Ways to Reduce Friction - Fleas are Good for Something!
Scenarios for reducing friction are presented including hydroplanes and air tracks. Even a flea can move a very heavy book if the friction is near zero (as demonstrated).