1. Friction Lesson 4

2. I. Definition Friction is a force that always exists between any two surfaces in contact with each other Friction is a force that always exists between any two surfaces in contact with each other It is a force that opposes motion It is a force that opposes motion

3. II. Two Theories on Friction Nobody is exactly sure why friction acts the way it does… a. minute (tiny) imperfections in the surfaces grinding against each other. (two pieces of sandpaper) - all surfaces have tiny bumps which will grind past other bumps on other surfaces and cause friction. - all surfaces have tiny bumps which will grind past other bumps on other surfaces and cause friction. b. small electrostatic attractions between atoms of the two surfaces, pulling on each other. - electrons in one of the surfaces attract to the protons in the other surface. Billions of these attractions between the electrons and protons of the two objects cause them to stick to each other somewhat causing friction - electrons in one of the surfaces attract to the protons in the other surface. Billions of these attractions between the electrons and protons of the two objects cause them to stick to each other somewhat causing friction

4. III. Two kinds of friction 1. Static friction - exists between two surfaces that are not moving relative to each other. - exists between two surfaces that are not moving relative to each other. 2. Kinetic friction - exists between two surfaces that are moving relative to each other. - exists between two surfaces that are moving relative to each other.

5. Static Friction In any situation, the static friction is greater than the kinetic friction. In any situation, the static friction is greater than the kinetic friction. Did you notice that you were pushing harder, and harder, and HARDER, until suddenly it was like glue that was holding it to the floor snapped? Then, it felt easier to push the object than it did just to get it started. Did you notice that you were pushing harder, and harder, and HARDER, until suddenly it was like glue that was holding it to the floor snapped? Then, it felt easier to push the object than it did just to get it started.

6. Kinetic Friction ● When it was still, you were trying to overcome the static friction (bigger force). ● When it was still, you were trying to overcome the static friction (bigger force). ● When it finally started to move, you were now pushing against the kinetic friction (smaller force) ● When it finally started to move, you were now pushing against the kinetic friction (smaller force)

7. IV. Two factors that affect frictional force the magnitude of the normal force acting on the object by the surface the magnitude of the normal force acting on the object by the surface the nature of the interaction between the atoms and electrons of the object and the atoms and electrons of the surface the nature of the interaction between the atoms and electrons of the object and the atoms and electrons of the surface

8. V. Coefficient of Friction The interaction between the surfaces is represented by a number μ (pronounced “mu”) The interaction between the surfaces is represented by a number μ (pronounced “mu”) called the coefficient of friction (an indicator of the “stickiness” of the interaction between two surfaces) called the coefficient of friction (an indicator of the “stickiness” of the interaction between two surfaces) Properties: Properties: the higher the coefficient, the stickier the interaction, the greater the frictional force. the higher the coefficient, the stickier the interaction, the greater the frictional force. For frictionless interactions, which are only theoretical, μ = 0. For frictionless interactions, which are only theoretical, μ = 0. coefficient of friction is unitless. coefficient of friction is unitless.

9. Friction Coefficients Surfaces μs μk Surfaces μs μk steel on steel 0.74 0.57 steel on steel 0.74 0.57 aluminum on steel 0.61 0.47 aluminum on steel 0.61 0.47 copper on steel 0.53 0.36 copper on steel 0.53 0.36 rubber on concrete 1.0 0.8 rubber on concrete 1.0 0.8 wood on wood 0.25 – 0.5 0.2 wood on wood 0.25 – 0.5 0.2 glass on glass 0.94 0.4 glass on glass 0.94 0.4 waxed wood on wet snow 0.14 0.1 waxed wood on wet snow 0.14 0.1 waxed wood on dry snow - 0.04 waxed wood on dry snow - 0.04 metal on metal (lubricated) 0.15 0.06 metal on metal (lubricated) 0.15 0.06 ice on ice 0.1 0.03 ice on ice 0.1 0.03 teflon on teflon 0.04 0.04 teflon on teflon 0.04 0.04 synovial joints in humans 0.01 0.003 synovial joints in humans 0.01 0.003

10. The coefficient of static friction (μs) is used to calculate the frictional force which must be overcome to start the object moving. The coefficient of static friction (μs) is used to calculate the frictional force which must be overcome to start the object moving. The kinetic coefficient of friction (μk) is used to calculate the frictional force when an object is already in motion. The kinetic coefficient of friction (μk) is used to calculate the frictional force when an object is already in motion.

11. VI. Calculating Friction: Friction always acts in the direction opposite to the motion of the object. Friction always acts in the direction opposite to the motion of the object. - Just look at the direction the object is traveling. The direction of the force due to friction will be exactly 180° opposite. - Just look at the direction the object is traveling. The direction of the force due to friction will be exactly 180° opposite. - Friction is also proportional to the normal force, which is how we'll be able to calculate it. - Friction is also proportional to the normal force, which is how we'll be able to calculate it.

13. Example 1 What friction force must be overcome to start a 50 kg object sliding across a horizontal surface when the static coefficient of friction is 0.35? What friction force must be overcome to start a 50 kg object sliding across a horizontal surface when the static coefficient of friction is 0.35?

14. Example 2 A 400 N object is pulled over a horizontal surface. If the coefficient of kinetic friction is 0.23 and the applied force is 120 N, what is its acceleration? A 400 N object is pulled over a horizontal surface. If the coefficient of kinetic friction is 0.23 and the applied force is 120 N, what is its acceleration?

15. Example 3 A 10 kg box is dragged over a horizontal surface by a force of 40 N. If the box moves with a constant speed of 0.5 m/s, what is the coefficient of kinetic friction for the surface? A 10 kg box is dragged over a horizontal surface by a force of 40 N. If the box moves with a constant speed of 0.5 m/s, what is the coefficient of kinetic friction for the surface?

16. Example 4 A 10 kg box is dragged across a level floor with a force of 60 N. The force is applied at an angle of 30° above the horizontal. If the coefficient of friction is 0.20, what is the acceleration of the box? A 10 kg box is dragged across a level floor with a force of 60 N. The force is applied at an angle of 30° above the horizontal. If the coefficient of friction is 0.20, what is the acceleration of the box?