1. Friction is a type of force between two touching surfaces. Section 2: Friction K What I Know W What I Want to Find Out L What I Learned

2. Essential Questions What is the friction force? How do static and kinetic friction differ? Friction Copyright © McGraw-Hill Education

3. Review force New kinetic friction static friction coefficient of kinetic friction coefficient of static friction Friction Copyright © McGraw-Hill Education Vocabulary

4. Friction Copyright © McGraw-Hill Education Friction is a force that opposes the motion of two surfaces that are in contact with each other. There are two types of friction—kinetic friction and static friction. Static friction is the force exerted on one surface by another when there is no motion between the two surfaces. For example, the force applied to the couch below is balanced by the static friction force. Kinetic and Static Friction Animation

5. Friction Copyright © McGraw-Hill Education If you push hard enough, the couch will begin to move and kinetic friction will act on it. Kinetic friction is exerted on one surface by another when the two surfaces rub against each other because one or both of them are moving. Kinetic and Static Friction

6. Friction Copyright © McGraw-Hill Education Frictional force depends on the materials that the surfaces are made of. The different lines correspond to dragging a block along different surfaces. Kinetic and Static Friction

7. Friction Copyright © McGraw-Hill Education The slope of this line is the coefficient of kinetic friction (μ k ) between the two surfaces and relates the frictional force to the normal force, as shown below. Notice that the normal force (F N ) also plays a role in the size of the friction force. Similarly, μ s is the coefficient of static friction between the two surfaces. μ s F N is the maximum static friction force that must be overcome before motion can begin. Kinetic and Static Friction Kinetic Friction Force Static Friction Force

8. Friction Copyright © McGraw-Hill Education Kinetic and Static Friction

9. Friction Copyright © McGraw-Hill Education Kinetic and Static Friction EVALUATE THE ANSWER Force is in newtons, so the units are correct. Use with Example Problem 3. Problem A child drags a heavy, rubber-soled shoe by its laces across a sidewalk at a constant speed of 0.35 m/s. The shoe has a mass of 1.56 kg and the coefficient of kinetic friction is 0.65. What is the force exerted by the child? (Assume the sole is in contact with the sidewalk, not bouncing around and that the child pulls horizontally.) Response SKETCH AND ANALYZE THE PROBLEM Draw a vector diagram. List the knowns and unknowns. KNOWN v = 0.35 m/s m = 1.56 kg μ k = 0.65 UNKNOWN F child = ? F child FNFN FgFg FfFf vgvg SOLVE FOR THE UNKNOWN Because the shoe is not accelerating in the y-direction, F net, y = 0. Thus, the normal force and the shoe’s weight are equal in magnitude. Because the shoe is not accelerating in the x-direction, F net, x = 0. Thus, the force the child exerts and the frictional force are equal in magnitude.

10. Friction Copyright © McGraw-Hill Education Kinetic and Static Friction Use with Example Problem 4. Problem If the child in the previous Additional In-Class Example pulls with an extra 2.0 N in the horizontal direction, what will be the acceleration of the shoe? Response SKETCH AND ANALYZE THE PROBLEM Draw a force diagram. List the knowns and unknowns. KNOWNUNKNOWN F child = 11.9 Na = ? m = 1.56 kg μ k = 0.65 SOLVE FOR THE UNKNOWN Find the net force in each direction. F net, y = 0 and F N = mg, as it did in the previous example. F f is same as it was in the previous example, so F net, x = F child − F f = 2 N. Use Newton’s second law. F child FNFN FgFg FfFf a EVALUATE THE ANSWER 1.3 m/s 2 is about (1/8)g, so it is a reasonable acceleration.

11. Friction Copyright © McGraw-Hill Education Review Essential Questions What is the friction force? How do static and kinetic friction differ? Vocabulary kinetic friction static friction coefficient of kinetic friction coefficient of static friction