Friction F f is a force that acts between 2 touching objects always parallel to the 2 surfaces in contact always opposite the direction of (attempted) motion so make sure you make it negative when placed in the F net eq’n

3 Types of Friction 1. Static - F fs – opposes the start of motion For any 2 given surfaces, it has a range of values: 0 < F fs < max when motion begins 2. Sliding (kinetic) - F fk – opposes the actual motion Has a constant value for any 2 given surfaces Contrary to popular belief, does NOT depend on Amount of surface area touching Relative speed between the 2 surfaces 3. Rolling – like with a ball – more in Ch 11

so the amount of friction depends on 2 things: 1. μ is the Greek letter mu it represents the coefficient of friction “the nature of the 2 surfaces in contact…” it has no units: μ = F f / F N would cancel the only units of Newton/Newton its value is determined experimentally by the 2 materials in contact (see chart) Note: static friction is generally greater than sliding friction for any 2 surfaces. This explains why it’s harder to get an object moving than it is to keep it moving! (Not due to inertia!)

2 Surfaces in Contact μ for static friction (μ s ) μ for kinetic friction (μ k ) Wood on wood Up to.5.2 Ice on ice Up to.2.03 Lubricated steel on steel Up to.2.07 Dry steel on steel Up to.9.6 Rubber on dry concrete Up to Rubber on wet concrete Up to.8.5 Rubber on dry asphalt Up to.7.5 Rubber on wet asphalt Up to Teflon on Teflon Up to.04.04

(so the amount of friction depends on 2 things:) 2. F N is the normal force Recall this is the perpendicular supporting force of a surface that’s under an object Determined by how much the 2 surfaces are pressed together as they try to move across each other So while F N is not the weight, the weight will often play some role here This is a magnitude only equation – it only determines the size of the force of friction. F f ’s direction is always negative, but we don’t deal with that when using this equation.

Does Friction only occur between solids? No, also between fluids – anything that flows liquids and gases are both fluids Ex: Air resistance …which we will get into in greater detail a little later in this chapter.