Where is friction needed?

Reading Assignment 5.3: Which of the following is a friction force? A.The component of a contact force parallel to the surface B.The component of a contact force perpendicular to the surface. C.Any contact force between two surfaces. D.Any force if surfaces slide on each other.

Characteristics of friction Two surfaces in contact An applied force With a component parallel to the contact surface F Normal force Reaction force to normal force

Characteristics of friction F appl v=0 Net force on block? fsfs F appl fsfs Static friction: no relative motion between surfaces F appl F appl > f s,max F appl = f s

Characteristics of friction F appl fkfk Kinetic friction: surfaces slide over each other v=const F appl = f k

Characteristics of friction F appl fkfk Kinetic friction: surfaces slide over each other v increasing F appl > f k

Characteristics of friction F appl fkfk Kinetic friction: surfaces slide over each other v decreasing F appl < f k

Comparison of static and kinetic friction Both friction forces oppose the applied force. F f Both friction forces show connection with the normal force. Empirical formula:

Transition between static and kinetic friction Friction force Applied force static motion See table in book for friction coefficients

Reading Assignment 5.3: Which of the following is important in walking? A.Kinetic friction between shoes and ground. B.Static friction between shoes and ground. C.Cling force between shoes and ground. D.The forward push of inertia.

Pushing and pulling cardboard boxes of equal mass: In which of these situations is the frictional force the largest? Smallest? Explain.

v = 0.5 m/s v = 10 m/s

You notice that a brick is sliding down a board at constant speed, if the board is held at an angle of 24 degrees. a)Determine the coefficient of kinetic friction. b)The static friction coefficient for brick on wood is How much can you tilt the board with a resting brick on it before it will start to slide?

Stacking crates One of two identical 200-kg crates is dragged along the floor at a constant speed by a horizontal force of 200 N. How much force would it take to pull them both at some constant speed if a) they are tied together one behind the other? b) the second is stacked on top of the first? c) What is the direction of the static friction force on the top crate?

Holding a box You hold a big heavy box between your outstretched hands. The friction coefficient is  s =0.5. With how much force do you need to press your arms together in order to not let the box of mass 25 kg slip down?

Your car is stuck in some deep snow. Why should you take your foot off the gas pedal? How can that help to get traction back?