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Friction

Friction. And Forces in Two Dimensions. Review. A force is a push or pull. Newton’s 3 Laws of Motion: Objects resist changes in velocity unless acted upon by a net force. The magnitude of an object’s acceleration depends on the net force and inversely on the object’s mass. a = F net / m

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Friction

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  1. Friction And Forces in Two Dimensions

  2. Review • A force is a push or pull. • Newton’s 3 Laws of Motion: • Objects resist changes in velocity unless acted upon by a net force. • The magnitude of an object’s acceleration depends on the net force and inversely on the object’s mass. • a = Fnet / m • For every force there is an equal and opposite counter-force.

  3. Friction • A force that opposes motion. • It was once thought that friction was caused by surface roughness. • In this model, microscopic nooks and bumps on the surface of an object lock and slip against similar features on another surface. • It is now known that this model is incorrect. • Friction is caused by brief electromagnetic interactions between charged particles in two surfaces.

  4. Friction • Two important types of friction: • Static friction • Prevents motion from beginning • Kinetic friction • Prevents motion from continuing • Static friction is usu. stronger than kinetic friction. • This is why, when you’re pushing a heavy object across the floor, it’s harder to start it moving than it is to keep it moving.

  5. Friction • Ff =  FN • Ff = Friction (N) •  = Coefficient of friction (no units) • FN = Normal force = mg cos • Coefficient of friction • Unique for every pair of substances and set of conditions. • Different CoF for static and kinetic friction.

  6. Coefficients of Friction

  7. Coefficient of Friction • CoF must be determined experimentally for any pair of surfaces. •  = Ff / FN

  8. Sample Problem • The coefficient of static friction between two blocks of wood is 0.500. What applied force is necessary to start a 250.-kg wooden box sliding along a horizontal wooden surface? • Fa must be equal to or greater than Ff,static. • Ff =  FN • FN = Fw = mg • FN = (250. kg)(9.81 m/s2) • FN = 2450 N • Ff = (0.500)(2450 N) • Ff = 1230 N

  9. Sample Problem 2 • A box with a mass of 40.0 kg requires an applied force of 250. N to keep it moving at a constant speed along a horizontal surface. What is the CoF between the box and the floor? • Ff =  FN • Ff =  mg • (250. N) =  (40.0 kg)(9.81 m/s2) • (250. N) =  (392 N) •  = 0.638

  10. In lab today • We will calculate the coefficient of friction between a shoe and the surface of a ramp. • You will volunteer the use of your shoes! • Place the shoes on a ramp. • Incline the ramp until the shoe begins to slide. • Measure the angle at which the shoe begins to slide. • Calculate the coefficient of friction using some simplifying math. • Repeat the procedure but first wet the shoe’s sole and the board.

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