BELLWORK 2/06/18 If you were to travel to the moon, A. Would your mass or weight change? Why? B. Would it be less or more on the moon? Fg is also defined as ___________ and the formula for calculating weight (Fg) is _____________?

Force of Friction

Friction Any force that resists motion It involves objects that are in contact with each other. Some scientists believe that friction is caused by uneven surfaces of the touching objects – when rubbed together resistance is offered. Experiments have shown that tiny particles are actually torn from one surface and imbedded in the other.

There are two types of friction: Static Friction (Fs) – the frictional force present just before motion begins. Starting motion is when Static Friction is at is maximum. Kinetic Friction (Fk) – the frictional force present with motion Note: Static Friction is usually higher than Kinetic Friction

Free Body Diagram FN = Fg Ff = uFN Fg (weight) = mg Normal Force FN Friction Force ff Applied Force F Gravity Force Fg Fg (weight) = mg FN = Fg Ff = uFN

CoefCoefficient of Frictiontrs Coefficient of Friction- quantity that expresses the amount of frictional forces depending on the surface the object is sliding across Represented by the symbol u (mu) Ratio of forces. Ex: uk is the ratio of Fk to Fn

Ta Material Coefficient of Static Friction S Coefficient of Kinetic Friction k Rubber on Glass 2.0 Rubber on Concrete 1.0 0.8 Steel on Steel 0.74 0.57 Wood on Wood 0.25 – 0.5 0.2 Metal on Metal 0.15 0.06 Ice on Ice 0.1 0.03 Synovial Joints in Humans 0.01 0.003

Copy all of the Formulas from Chapter 4 from Page 855 on your index card. Sub-title these formulas: “Laws of Motion”

CLASSWORK Page 139 #’s 1-3