1. Friction Friction is the force that opposes motion whenever one surface moves or tends to move with reference to another surface. Static Friction (F s ) the force that tends to prevent a stationary object from starting to move i.e. the amount of force that prevents your textbook from sliding off your desk FaFa FsFs FaFa FsFs F a ˂ F s  not moving F a = F s  just starting to move FaFa FsFs F a ˃ F s  accelerating

2. Kinetic Friction (F k ) the force that acts against an object’s motion in a direction opposite to the direction of motion – the force that opposes motion once the force overcomes that static friction the object begins moving and kinetic friction replaces static friction FaFa FkFk **harder to start something moving than keep it moving F s ˃ F k F a = F k if already moving  no acceleration

3. Factors That Affect Friction 1.type of friction static friction is slightly greater than kinetic friction sliding friction is much greater than rolling friction 2.object’s mass as the mass of an object increases, the friction between the object and the surface on which it is moving increases 3.types of surface in contact rough surfaces tend to have more friction than smooth surfaces Friction Applet 3.3 Prac. #2-4