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Friction
ACTIVITY!!!
Types of Fingerprints
Arches Loops Whorls
What is the purpose of fingerprints?
Friction – a force that opposes motion between two surfaces
Caused by hills and valleys of the surfaces touching each other
Microscopic View of Scissors
Amount of Friction
Depends on roughness of surface and force pressing them together
• The rougher the surface, the greater the friction
Bowling ball vs. Bowling ball on lane on grass
• The greater the force, the greater the friction
Changing how much of the surfaces come in contact does not change the
amount of friction.
Why?
Two Types of Friction
Kinetic Friction Static
Friction
Kinetic friction – friction between moving surfaces
Depends how the surfaces move
Sliding – surfaces slide
Rolling – surfaces roll
Sliding Vs. Rolling
• Usually the force of sliding kinetic friction is greater than the force of rolling kinetic friction
When would sliding be easier?
Static friction – a force does not cause the object to move
• The force of static friction balances the force applied
Static Friction
• As soon as the object starts moving, static friction changes to kinetic friction
How does friction affect a car?
Harmful Friction:
Helpful friction
Try It!
Ways to reduce friction:
• use a lubricant
Lots of lubricants are liquids, but they can be solids and gasses as well
Ways to reduce friction:
• Switch from sliding to rolling
Ways to reduce friction:
• Make surfaces smoother
Ways to Increase Friction:
• Make surfaces rougher
Ways to Increase Friction:
• Increase the force
• Turbulence factor• An interesting aspect of fluid friction is that setting up tiny areas of
turbulence on the surface will reduce the friction even more. A good example of that are the dimples on the surface of a golf ball. If an ball had a completely smooth surface, it would not fly as far as an identical ball with a dimpled surface. The reason is that each dimple creates a small area of turbulence on the surface. This means the air is then flowing over air in that area and not along the surface of the ball, thus reducing the resistive friction.
• Dimples on golf ball reduce surface friction• But note that if the dimples were too large or deep, then the
turbulence would cause the ball to slow down. Dimple dimensions and locations are very important in the design of golf balls.