Friction There are many forms of friction. This lesson introduces the force laws for static friction, kinetic friction, and rolling friction. Students learn the meaning and typical range of values for the coefficients of friction. In the investigation, students determine the coefficients of static and kinetic friction between two surfaces.

Objectives Calculate friction forces from equation models for static, kinetic, and rolling friction. Solve one-dimensional force problems that include friction. The lesson objectives describe what a student should know and be able to do.

Assessment A box with a mass of 10 kg is at rest on the floor. The coefficient of static friction between the box and the floor is 0.30. Estimate the force required to start sliding the box. This first assessment is keyed to the first objective: calculate friction forces from equation models for static, kinetic, and rolling friction. It will be repeated at the end of the lesson, followed by the answer.

Assessment A 500 gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. Draw a free-body diagram for the puck and calculate the magnitude of each force. How long will it take the puck to skid to a stop? The second assessment is keyed to the second objective: solve one-dimensional force problems including friction. It will be repeated at the end of the lesson, followed by the answer.

Physics terms coefficient of friction static friction kinetic friction rolling friction viscous friction air resistance This slide lists new or important terms for this lesson.

Equations Models for friction kinetic friction static friction rolling friction Models for friction The friction force is approximately equal to the normal force multiplied by a coefficient of friction. This slide lists new equations for this lesson.

What is friction? Friction is a “catch-all” term that collectively refers to all forces which act to reduce motion between objects and the matter they contact. Friction often transforms the energy of motion into thermal energy or the wearing away of moving surfaces.

Kinds of friction The term ‘friction” is used to describe a variety of resistive forces. Air friction, for example, is also referred to as air resistance, and as drag. Another type of friction that will be treated in this lesson but is not mentioned in the slide is static friction.

Kinetic friction Kinetic friction is sliding friction. It is a force that resists sliding or skidding motion between two surfaces. If a crate is dragged to the right, friction points left. Friction acts in the opposite direction of the (relative) motion that produced it. This slide describes the direction of the friction force. For kinetic (sliding friction) the direction is easy to determine or describe.

Which takes more force to push over a rough floor? Kinetic friction Which takes more force to push over a rough floor? This slide begins a set of slides that develop the equation model for the magnitude of friction.

Friction and the normal force The board with the bricks, of course! The simplest model of friction states that frictional force is proportional to the normal force between two surfaces. If this weight triples, then the normal force also triples—and the force of friction triples too. Point out that the normal force is not always equal to the weight of an object. If you push a book against the wall and slide it upward, the normal force will be equal to the horizontal component of your push.

A model for kinetic friction The force of kinetic friction Ff between two surfaces equals the coefficient of kinetic friction μk times the normal force FN. direction of motion But what is this coefficient of friction, μk? Ask the students what they think this new constant could be related to.

The coefficient of friction The coefficient of friction is a constant that depends on both materials. Pairs of materials with more friction have a higher μk. direction of motion The μk tells you how many newtons of friction you get per newton of normal force. Do you see why μk has no units?

A model for kinetic friction The coefficient of friction μk is typically between 0 and 1. direction of motion When μk = 0 there is no friction. When μk = 0.5 the friction force equals half the normal force. When μk = 1.0 the friction force equals the normal force. Ask for an example of a high coefficient of friction, and a low coefficient of friction. How do you lower the friction in a machine?

Calculating kinetic friction Consider a 30 N brick sliding across a floor at constant speed. What forces act on the block? Draw the free body diagram.

Calculating kinetic friction Consider a 30 N brick sliding across a floor at constant speed. What is the friction force on the brick if μk = 0.5?

Calculating kinetic friction Consider a 30 N brick sliding across a floor at constant speed. The force F needed to make the board slide at constant speed must also be 15 N. According to the model, the friction force will be 15 N at any (non-zero) speed. But IF the speed is constant, there must be an applied force equal to the friction.

Static friction Static friction is gripping friction. It is a force that prevents relative motion between surfaces in contact with each other. Without static friction between your feet and the floor, you could not walk or run. Your feet would slip. Without static friction between your tires and the road, you could not start or stop a car. Note: Static friction does not actually prevent motion. It prevents relative motion. Demonstration: place a block on top of a pad of a paper. Move the block back and forth by sliding the pad back and forth. it is static friction that keeps the block at rest relative to the pad.

Static friction Static friction prevents this crate from sliding when pushed . . .

Static friction Static friction prevents this crate from sliding when pushed . . . . . . until the pushing force is greater than the maximum static friction force available.

Static friction How much static friction acts in case a? In case b? Point out to students that in a and b the crate remains at rest, so the net force on it must be zero. Therefore the static friction 120 N in a, and 160 N in b. The static friction has a maximum value of 200 N. A good analogy: static friction is like money in the bank; you only take out exactly what you need. If only 120 N is needed to keep an object at rest, this is exactly how much static friction will act on the object. But there is a maximum amount available in the bank.

Static friction How much static friction acts in case a? 120 N In case b? 160 N The crate is at rest so the net force must be zero. The static friction increases exactly as needed to keep the box at rest.

Static friction How much static friction acts in case a? 120 N In case b? 160 N What is the maximum static friction available? Sliding friction is typically LESS than the maximum static friction, so once the crate gets moving the force needed to KEEP it moving drops to 160 N.

Static friction How much static friction acts in case a? 120 N In case b? 160 N What is the maximum static friction available? 200 N Once the maximum static friction is exceeded, the crate begins to move. Sliding friction is typically LESS than the maximum static friction, so once the crate gets moving the force needed to KEEP it moving drops to 160 N.

A model for static friction The maximum static friction force Ff between two surfaces is the coefficient of static friction μs times the normal force FN. direction of applied force When μs = 0 there is no friction. When μs = 0.5 the maximum friction force equals half the normal force. When μs = 1.0 the maximum friction force equals the normal force. It is actually possible but unusual to have a coefficient of friction greater than one.

Assessment A box with a mass of 10 kg is at rest on the floor. The coefficient of static friction between the box and the floor is 0.30. Estimate the force required to start sliding the box. This first assessment is keyed to the first objective: calculate friction forces from equation models for static, kinetic, and rolling friction. The answer appears on the next slide.

Assessment A box with a mass of 10 kg is at rest on the floor. The coefficient of static friction between the box and the floor is 0.30. Estimate the force required to start sliding the box. The required force is about 29 N. This first assessment is keyed to the first objective: calculate friction forces from equation models for static, kinetic, and rolling friction. The answer appears on the next slide.

Assessment A 500-gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. Draw a free-body diagram for the puck. The second assessment is keyed to the second objective: solve one-dimensional force problems including friction. The answer appears on the next slides.

Assessment A 500-gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. Draw a free-body diagram for the puck. direction of motion

Assessment A 500-gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. Draw a free-body diagram for the puck and calculate the magnitude of each force. direction of motion

Assessment A 500-gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. How long will it take the puck to skid to a stop? Hint: What is the acceleration of the puck? direction of motion

Assessment A 500-gram puck is sliding at 20 m/s across a level surface. The coefficient of kinetic friction between the puck and surface is 0.20. How long will it take the puck to skid to a stop? direction of motion