Lecture 9: Forces & Laws of Motion

Questions of Yesterday You 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 F c) greater than F d) 0 1b) Are other forces acting on the book in the horizontal direction? a) YES b) NO c) not enough information to know 3) 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 acceleration b) opposite the direction of the truck’s acceleration c) the net force is zero

Newton’s First Law An 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?

Friction An object moving on a surface or through a viscous medium encounters resistance to motion from the surface or medium FRICTION The resistive force that acts between two surfaces to oppose motion Friction ALWAYS acts in a direction opposite the motion of the object FAFA Friction = f motion

Friction FRICTION The resistive force that acts between two surfaces to oppose motion FAFA Friction = f FgFg N

Static Friction FgFg N You apply a horizontal force F A to try to move the block across the table but it doesn’t move… what forces are acting on the block? FAFA fsfs

Static Friction FgFg N Friction force that acts on object at rest in a direction opposite the applied force to keep it stationary FAFA fsfs How do the magnitudes of F A and f s compare?

Static Friction FgFg N F A = -f s Object is at rest -> NO Acceleration, NO net force FAFA fsfs What if the block is moving?

Kinetic Friction FgFg N Friction force that acts on a moving object in a direction opposite to the objects motion FAFA fkfk How do F A and f k compare? motion

Kinetic Friction FgFg N constant velocity motion from F A constant acceleration motion from F A FAFA fkfk FgFg N FAFA fkfk F A = -f k F A > -f k

Friction & the Normal Force Which box would you rather push across the floor? Why? Friction force is only other force acting in horizontal direction! 10 kg 100 kg FgFg N FAFA F g and N shouldn’t affect horizontal motion! FAFA N FgFg

Friction Force is proportional to Normal Force acting on an object…. which typically depends on the weight of the object FgFg N FAFA 10 kg 100 kg FAFA N FgFg fkfk fkfk Friction & the Normal Force

FgFg N FAFA 10 kg 100 kg FAFA N FgFg fkfk fkfk f s ≤  s N f k =  k N Friction & the Normal Force  k,  s = coefficients of kinetic, static friction  k,  s are experimentally determined values that depend on the nature of the surface

FAFA M f s ≤  s N f k =  k N Friction & the Normal Force You apply a force of magnitude F A 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 )? FAFA M FAFA M 30 o

Free Body Diagrams If 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  )?  M

Friction: Practice Problem What 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 kg 10 kg  s = 0.5  k = 0.3

Friction: Practice Problems A 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 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 Problems A firefighter of mass 80 kg slides down a vertical pole with an acceleration of 4 m/s 2. What is the friction force that acts on the firefighter?

Questions of the Day 1) A student pushes her physics book across a flat table. Another student pushes his book up a 30 o 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 table b) the book on the inclined plane c) 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) downwards c) away from the chalkboard d) into the chalkboard