“Rain, Steam, and Speed – The Great Western Railway” (1844) FRICTION Vocabulary: friction normal force coefficient of friction (μ) coefficient of kinetic friction (μk) coefficient of static friction (μs) “Rain, Steam, and Speed – The Great Western Railway” (1844) J.M.W. Turner This topic can be found on pp. 135 – 139 of your textbook.

(p)re:think What is friction? What causes friction? Draw it! Let’s compare.

What is friction? Friction is a force that opposes motion. sliding friction fluid friction When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Weight and mass are often confused. Students learned earlier that mass was the amount of matter in an object and weight was the force of gravity, but they often still confuse the issue. When eliciting their responses, ask them to discuss appropriate units for each. You might discuss “weightlessness” and ask if objects can be massless as well. Friction is often confused with heat or thermal energy. Students likely will think of friction as being related to many of the quantities listed above. rolling friction

What is friction? We are going to only discuss sliding friction during this unit. sliding friction When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Weight and mass are often confused. Students learned earlier that mass was the amount of matter in an object and weight was the force of gravity, but they often still confuse the issue. When eliciting their responses, ask them to discuss appropriate units for each. You might discuss “weightlessness” and ask if objects can be massless as well. Friction is often confused with heat or thermal energy. Students likely will think of friction as being related to many of the quantities listed above.

What causes friction? glass aluminum foil How do these highly magnified images of glass and aluminum foil fit in with your predictions of the cause of friction from earlier?

What causes friction? When two surfaces slide across each other, microscopic imperfections between the surfaces cause the two surfaces to exert opposite forces on one another. movement

Friction is everywhere!! Watch this

What factors do you think affect friction (i. e What factors do you think affect friction (i.e., what can increase or decrease the force of friction?)

Lab Time!!!! You will experimentally determine which factors affect the force of friction.

Factors Affecting Friction The normal force acting on an object, FN. The normal force is a function of the weight of the object experiencing the friction. The nature of the surfaces in contact. This is quantified using a coefficient of friction, µ. The higher the value of µ, the higher the friction force.

Factors Affecting Friction Which will have a larger value of ?

Factors Affecting Friction  is the ratio between the friction force between two objects and the normal force. It must be experimentally determined.

Calculating the Force of Friction (Ff) Ff is directly proportional to Fn (normal force) Coefficient of friction (): Determined by the nature of the two surfaces s is for static friction (force is applied, but object does not move.) k is for kinetic friction (force is applied and object is moving.) s > k Point out to students that Ff is the general term for both static friction (Fs) and kinetic friction (Fk).

Typical Coefficients of Friction Values for  have no units and are approximate (why?) Point out that static is greater than kinetic for each example. Also explain that the coefficient is generally less than 1 but there could be sticky surfaces where the frictional force was greater than the normal force. This would lead to coefficients greater than 1. Why do you think μs is always greater than μk?

Static vs. Kinetic Friction When two objects are stationary, the “bumps” on each object settle into one another. To get the objects moving, you have to get these bumps out from one another. μk When two objects are moving, the “bumps” do not have time to settle into one another. Instead, they glide over one another offering less resistance. Point out that static is greater than kinetic for each example. Also explain that the coefficient is generally less than 1 but there could be sticky surfaces where the frictional force was greater than the normal force. This would lead to coefficients greater than 1.

What about surface area? Friction force does not depend on the area of contact. Each block has the same FN and µ!

Review: Weight and Mass Mass is the amount of matter in an object unit = Kilograms Weight is a measure of the gravitational force on an object unit = Newtons Depends on the acceleration of gravity Weight = mass  acceleration of gravity W = mag where ag = 9.8 m/s2 on Earth Mention that weight is less on the moon because ag on the moon is 1.6 m/s2 . Reinforce that converting between mass and weight is simple, just multiply or divide by 9.81 m/s2 . Point out that each kg has a weight of 9.81 N on Earth.

Review: Normal Force Force on an object perpendicular to the surface (Fn) It may equal the weight (Fg), as it does in the picture of the TV. It does not equal the weight (Fg) if the surface is angled, as in the second example Fn = mg cos  Point out that the equation for normal force applies to the first example also. Because cos(0)=1, the equation reduces to Fn = mg when the forces are directly opposite one another.

Classroom Practice Problem A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to just get it moving. Once it is moving, it only requires 50. N of horizontal force to keep it moving. Find the coefficient of static friction and coefficient of kinetic friction between the crate and the floor. Draw a free-body diagram and use it to find: the weight the normal force (Fn) the force of friction (Ff) Find the coefficients of friction (both static and kinetic). Answer: s = 0.32 k = .21 This is a relatively simple example from the book (Sample Problem D). Ask students to follow the steps. It is easy to get the answer by skipping the free-body diagram, but they need this diagram to understand that normal force = weight, and the 75 N horizontal push is equal to the force of friction. More complicated problems (next slide) can’t be solved without a free- body diagram.

What is the frictional force in this scenario? EXPLAIN Classroom Practice Problem What is the frictional force in this scenario? EXPLAIN Fnormal Mass of the block is 500. g Force applied is 3.0 N.  = 0.75 The block is stationary Ffriction Fapplied These questions should help students understand that static friction balances the external force (F), so it increases and decreases as F increases and decreases. Eventually, F will be so large that the static frictional force (Fs) will no longer be able to balance it, and the net force will cause the object to slide. At this point, frictional forces become kinetic (see next slide). weight What is the maximum force that can be applied before the block starts to move? Answer: 3.7 N

Challenging problem A 1450 kg ‘64 Mustang travelling at 34 m/s hits its brakes and skids to a stop in 75 m. What is the coefficient of kinetic friction (μk) between the road and the car’s tires? Fnormal These questions should help students understand that static friction balances the external force (F), so it increases and decreases as F increases and decreases. Eventually, F will be so large that the static frictional force (Fs) will no longer be able to balance it, and the net force will cause the object to slide. At this point, frictional forces become kinetic (see next slide). Ffriction Answer: 0.79 Fgravity (weight)