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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

Friction

And Forces in Two Dimensions

review
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.
friction1
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.
friction2
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.
friction3
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.
coefficient of friction
Coefficient of Friction
  • CoF must be determined experimentally for any pair of surfaces.
    •  = Ff / FN
sample problem
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
sample problem 2
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
in lab today
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.