Phys Kinetic Static Friction

Honors Physics Lesson 3.6 Friction! Friction (Ff) ­ remember, friction the force applied by a surface an object is in contact with which either: • works against the motion of the object which is sliding across the surface. This is called Kinetic Friction (Ffk) • works against the attempted motion of the object sliding across the surface. This is called Static Friction (Ffs) 1 Static Friction Here’s what you need to know about the force of static friction: • Static friction only occurs if the object is not sliding across the surface and if the object WOULD slide if there was no static friction. For example: • Roy tries to slide a box across a table...static friction prevents the box from sliding. • A car which is speeding up, slowing down, or turning is able to do so because of static friction between the tires and the road. Static Friction is "lazy" & will exert as little force as possible to prevent the two objects from sliding across each other. This often means it makes the net force zero, but not always creates the net force • Ex: when changing velocity in a car, static friction that produces the acceleration. • When two objects rub together, there is amaximum amount of force static friction can exert. This amount varies depending on . The two surfaces result in • The two surfaces rubbing together what is called a "coefficient of static friction. " ( ) This is merely a ratio and has no units. In general: • Two surfaces which are "smoother" or "less slidey" will have a lower coefficient, closer to zero • Two surfaces which are "rougher" or "more coarse" will have a higher coefficient, closer to (and occasionally more than) one Coefficient • Materials: Teflon on Teflon 0.04 • Ice on Ice 0.10 • 0.60 • Tire on wet road 1.00 • Tire on dry road . • How tightly they are being pressed together • This is the Normal Force (F N) 2 Static Friction OR Example A 50 kg box sits on a floor. Roy pushes the box and discovers that he needs a force of (more than) 300 N to make it move. Find the coefficient of static friction between the box and the floor. (answer: 0.61) Example A 20 kg box sits on a floor. The coefficient of static friction between the floor and the box is 0.65. Make a FBD and describe what the box will do if Roy pushes the box horizontally with a force of: a. 80 N b. 100 N c. 130 N 3 Kinetic Friction Here’s what you need to know about the force of kinetic friction: • Kinetic friction only occurs if the object issliding across the surface. For example: • Roy slides a box across a table...kinetic friction will work against the box sliding. • A car which is spinning its wheels when speeding up, skidding when slowing down, or skidding when turning has kinetic friction acting on it. • When two objects are sliding across each other together, the amount of force kinetic friction exerts depends on . The two surfaces result in what • The two surfaces rubbing together is called a "coefficient of kinetic friction. " ( ) This is merely a ratio and has no units. In general: • Two surfaces which are "smoother" or "less slidey" will have a lower coefficient, closer to zero • Two surfaces which are "rougher" or "more coarse" will have a higher coefficient, closer to (and occasionally more than) one Coefficient • Materials: 0.04 • Teflon on Teflon 0.03 • Ice on Ice 0.40 • Tire on wet road 0.80 • Tire on dry road . • How tightly they are being pressed together ) • This is the Normal Force (F N 4 Kinetic Friction or Example A 20 kg box sits on a floor. The coefficients of static and kinetic friction between the floor and the box are 0.65 and 0.40 respectively. Make a FBD and describe what the box will do if Roy pushes the box horizontally with a force of: a. 80 N b. 100 N c. 130 N Example A 30 kg box sits on the floor. Roy discovers that if he pushes the box horizontally with a force of 300 N, the box accelerates at 4.5 m/s2. Find the coefficient of kinetic friction between the box and the floor. (Answer: 0.56) 5