Lecture 9: Forces & Laws of MotionPowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Lecture 9: Forces & Laws of MotionQuestions of YesterdayYou must apply a force F to push your physics book across your desk at a constant velocity.1a) The net force acting on the book is…a) F b) between 0 and Fc) greater than Fd) 01b) Are other forces acting on the book in the horizontal direction?a) YESb) NOc) not enough information to know3) A large crate is at rest in the bed of a truck. As the truck accelerates the crate remains at rest relative to the truck. In what direction is the net force on the crate?a) the same direction as the truck’s accelerationb) opposite the direction of the truck’s accelerationc) the net force is zeroNewton’s First LawAn object in motion will remain in its original state of motion unless acted on by an external force.But is that really what we observe??If objects in motion actually often come to a stop…and Newton’s first law really is true…What must be happening?FrictionAn object moving on a surface or through a viscous medium encounters resistance to motion from the surface or mediumFRICTIONThe resistive force that acts between two surfaces to oppose motionFriction ALWAYS acts in a direction opposite the motion of the objectFAFriction = fmotionFrictionFRICTIONThe resistive force that acts between two surfaces to oppose motionFAFriction = fFgNStatic FrictionFgNYou apply a horizontal force FA to try to move the block across the table but it doesn’t move…what forces are acting on the block?FAfsStatic FrictionFgNStatic Friction Friction force that acts on object at rest in a direction opposite the applied force to keep it stationaryFAfsHow do the magnitudes of FA and fs compare?Static FrictionFgNFA = -fsObject is at rest -> NO Acceleration, NO net forceFAfsWhat if the block is moving?Kinetic FrictionFgNKinetic FrictionFriction force that acts on a moving object in a direction opposite to the objects motion FAfkHow do FA and fk compare?motionKinetic FrictionFgNconstant velocity motion from FAconstant acceleration motion from FAFAfkFgNFAfkFA = -fkFA > -fkFriction & the Normal ForceWhich box would you rather push across the floor?Why?Friction force is only other force acting in horizontal direction! 10 kg100kgFgNFAFg and N shouldn’t affect horizontal motion!FANFgFriction Force is proportional to Normal Force acting on an object….which typically depends on the weight of the object FgNFA10 kg100kgFANFgfkfkFriction & the Normal ForceFgNFA10 kg100kgFANFgfkfkfs ≤ sNfk = kNFriction & the Normal Forcek, s = coefficients of kinetic, static frictionk, s are experimentally determined values that depend on the nature of the surfaceFAMfs ≤ sNfk = kNFriction & the Normal ForceYou apply a force of magnitude FA to move a box at constant velocity, what is the force of friction resisting the box’s motion in each case (in terms of m, g, k)?FAMFAM30o30oFree Body DiagramsIf the block is sliding down the inclined plane at a constant velocity what is the coefficient of kinetic friction between the block and the plane (in terms of m, g and )?MFriction: Practice ProblemWhat will the acceleration of the system be if… it is released from rest?it is set in motion with the 10.0 kg moving downward? 5 kg10 kgs = 0.5 k = 0.3Friction: Practice ProblemsA boy coasts down a hill on a sled, reaching a level surface at the bottom with a speed of 7.0 m/s. If the coefficient of friction between the sled’s runners and the snow is 0.050 and the boy and sled together weigh 600 N…How far does the boy travel on the level surface before coming to a stop?Friction: Practice ProblemsA firefighter of mass 80 kg slides down a vertical pole with an acceleration of 4 m/s2.What is the friction force that acts on the firefighter?Questions of the Day1) A student pushes her physics book across a flat table. Another student pushes his book up a 30o inclined plane. Assuming the coefficient of kinetic friction is the same in both cases, in which case is the force of friction acting on the book greater?a) the book on the flat tableb) the book on the inclined planec) the force of friction is the same in both cases 2) If you hold your physics book up against the chalkboard, in what direction is the force of friction directed?a) upwards b) downwardsc) away from the chalkboardd) into the