1. FRICTION

2. A force that opposes motion

3. Objectives… Explain why friction occurs. Explain why friction occurs. List the two types of friction, and give examples of each type. List the two types of friction, and give examples of each type. Explain how friction can be both harmful and helpful. Explain how friction can be both harmful and helpful.

4. What is friction? Is a force that opposes motion between two surfaces that are in contact Is a force that opposes motion between two surfaces that are in contact

6. When does friction occur? Friction occurs when two surfaces are in contact Friction occurs when two surfaces are in contact

8. Also… When a liquid or gas is pushed out of the way of a moving object When a liquid or gas is pushed out of the way of a moving object

9. Why does friction occur? Because the hills and valleys of a surface are in contact with the hills and valleys of other surfaces, this produce friction. Because the hills and valleys of a surface are in contact with the hills and valleys of other surfaces, this produce friction. This is obvious on rough surfaces such as asphalt or concrete, but what about “smooth” surfaces? This is obvious on rough surfaces such as asphalt or concrete, but what about “smooth” surfaces?

10. Friction at the Microscopic Level At the microscopic level ”smooth” surfaces show a lot of roughness.

12. Friction depends on… Roughness of surfaces in contact- Roughness of surfaces in contact- Force that presses objects one against the other; or their weights- Force that presses objects one against the other; or their weights- Or a combination of both. Or a combination of both.

13. What increases friction? Rough surfaces Rough surfaces Weight of objects Weight of objects

14. What does the amount of surfaces depend on? The amount of friction depends on how well the surfaces grip each other. Tires on a road have very high friction (they don't slip). Skates on ice have very low friction (they slip easily). The amount of friction depends on how well the surfaces grip each other. Tires on a road have very high friction (they don't slip). Skates on ice have very low friction (they slip easily). Tiresroad Tiresroad

15. In a liquid or a gas For a solid object moving through a liquid or gas, the amount of friction depends on the shape and surface area of the object. For a solid object moving through a liquid or gas, the amount of friction depends on the shape and surface area of the object. The viscosity (thickness or thinness) of the liquid or gas. The viscosity (thickness or thinness) of the liquid or gas.

16. Which one has less Friction?

17. TYPES OF FRICTION KINETIC friction of movement. KINETIC friction of movement. STATIC friction before movement. STATIC friction before movement.

18. What are the symbols for Friction? f s is the symbol for static friction- the3 friction to be overcome. f s is the symbol for static friction- the3 friction to be overcome. f k is the symbol for kinetic friction-the friction of movement. f k is the symbol for kinetic friction-the friction of movement. N is the symbol for the normal force. I will explain. N is the symbol for the normal force. I will explain.

19. What is the symbol for the Coefficient of Friction? The Greek letter Mu (pronounced myoo) The Greek letter Mu (pronounced myoo) Written µ (just like the micro symbol) Written µ (just like the micro symbol) µ k for kinetic friction µ k for kinetic friction µ s for static friction µ s for static friction Determined by the formula µ = Determined by the formula µ = Remember- Friction is always parallel to, and opposite to, the direction of movement. _F f _ FNFNFNFN

20. What is kinetic friction? Is the friction between two moving surfaces Is the friction between two moving surfaces

21. Sliding and Rolling

22. Static friction It occurs when a force is applied to an object but this object doesn’t move. It occurs when a force is applied to an object but this object doesn’t move.

23. Static friction Think of it as the amount of friction needing to be overcome before movement can occur.

24. Is friction good or bad? Friction can be good. When we walk we push backwards against the ground, the opposing force pushes us forwards. Without friction our feet would slip and walking would be impossible. Friction can be good. When we walk we push backwards against the ground, the opposing force pushes us forwards. Without friction our feet would slip and walking would be impossible.opposing forceopposing force

25. It is also good To stop your bicycle (brake system) To stop your bicycle (brake system)

26. But sometimes is bad… Soil erosion caused by wind (friction of a fluid). Soil erosion caused by wind (friction of a fluid). Unexpected friction can cause trips. Unexpected friction can cause trips. Uses up energy to overcome. Reduces mileage of vehicles. Uses up energy to overcome. Reduces mileage of vehicles. Lack of friction is dangerous too, you may fall if the floor is very slippy. Lack of friction is dangerous too, you may fall if the floor is very slippy.

27. Some ways to reduce friction

28. Some ways to increase friction

29. GO TO THIS LINK TO INCREASE YOUR KNOWLEDGE ABOUT FRICTION http://www.gcsescience.com/pfm32.htm http://www.gcsescience.com/pfm32.htm Video on Friction with math. Video on Friction with math. Video with PhET Video with PhET simulation PhET simulation Now go to PhET, Physics, Forces and Motions-Basic. Play with the Friction portion of the simulation.