Lecture 4 Friction School of Physics（第四讲摩擦物理学院）.pdf

Lecture 4 Friction and Newton’s 3rd law Pre-reading: KJF §4.8 Frictional Forces Friction is a force exerted by a surface. The frictional force is always parallel to the surface Due to roughness of both surfaces microscopic cold welding Proportional to normal force Always acts in the direction opposite to the slippage direction KJF §4.3, 5.5 2 Three kinds of friction: • Static friction: acts to prevent motion, so points opposite to the direction the object would move in the absence of friction • Kinetic friction: appears when an object slides across a surface; points opposite to the direction of motion • Rolling friction : one surface rolling over another KJF §4.3, 5.5 3 Coefficient of Friction µ μ is a measure of strength of friction.
 Scalar. No units - dimensionless • Depends on BOTH surfaces • Doesnt depend on size of contact area Usually: μ μ μ s k r Microscopic theory of friction very complex - cannot yet theoretically predict µ values KJF §5.5 4 Properties of friction A. Consider a person pushing a box on a horizontal surface with force F . If the box does not move p (static friction), then F = –F s p i.e. the static frictional force F adjusts itself to be exactly equal s and opposite to Fp B. As you increase the pushing force, eventual