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Friction

Friction. Friction- a force that opposes an impending or actual motion. Static Friction. Resists initial motion. Coefficient of static friction determines how much it resists and depends on the materials. μ s.

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Friction

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  1. Friction

  2. Friction- a force that opposes an impending or actual motion.

  3. Static Friction • Resists initial motion. • Coefficient of static friction determines how much it resists and depends on the materials. • μs

  4. As you apply more force to a block, the block will ultimately break loose and move. • It happens when you reach the threshold of motion. • The ToM exceeds the maximum possible value of the static friction

  5. 3 Insights to Static Friction • Ff is proportional to the normal force. • While proportional, it depends on the surface. • Ff is independent of the apparent size of the contact area between the two solid surfaces. (The surprising one).

  6. Ff= μsFN

  7. Example • A climber stands on the rock face of a mountain. The soles and heels of her boots have a static friction coefficient equal to 1.0. • What is the steepest slope she can stand on without slipping? • Assuming she has a static friction coefficient between pants and rock 0.3, what happens if she sits down to rest? Hint: tanθ= sinθ/cos θ

  8. Static friction is WHY WE CAN MOVE • It’s the driving force

  9. Grab a book and a protractor (any book in your bag will do). • Take your calculator (or mine---slackers) and place it on the cover. • Raise it until the calculator starts to slide. What’s the angle? • Calculate the coefficient of static friction between your calculator and your book.

  10. Kinetic Friction • Definition: the retarding force exerted on a sliding body in contact with a surface. • If in uniform motion (i.e, equilibrium), it will be equal and opposite to the applied force.

  11. Kinetic friction is generally less than static. Ff = μk FN

  12. Causes of Friction • The force that holds atoms and molecules together is electromagnetic. • Without this force, there’d be nothing. Not you, not me, not textbooks…etc.

  13. Only when atoms are close to each other will they experience an appreciable force attractive force. (adhesion) • Short range force • Negligible at distances equivalent to only about 4-5 atomic diameters. (1-2 x 10-10 m)

  14. Common objects may look smooth, but are jagged on a microscopic level. • Static friction arises out of the need to rip apart the areas of bonded contact. • To move the two surfaces, the welds must be torn.

  15. Keeping the surfaces separate helps reduce adhesion and friction. • Ex: baby oil, grease

  16. Adhesion is lessened if the two surfaces are already moving…that’s why μs > μk

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