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Friction

Friction. A resistance force usually caused by two surfaces moving past each other. Always in a direction that opposes the motion. Measured in Newtons Depends on surface texture and how hard the surfaces are pressed together.

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Friction

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  1. Friction • A resistance force usually caused by two surfaces moving past each other. • Always in a direction that opposes the motion. • Measured in Newtons • Depends on surface texture and how hard the surfaces are pressed together. • Surface texture determines the coefficient of friction (μ) which has no units. • Normal force measures how hard the surfaces are pressed together.

  2. Types of friction • Static friction is the force an object must overcome to start moving. • Kinetic friction is the force an object must overcome to keep moving. Static friction is always greater than kinetic friction!

  3. Calculating the Force of Friction Where f is the force of friction, μ is the coefficient of friction, and FN is the normal force For kinetic friction: For static friction:

  4. May the Net Force be with you • Total force acting on an object • Vector sum of all the forces • The unbalanced force referred to in Newton’s Law of Motion • Net force is equal to the mass of an object times the acceleration of that object.

  5. Force Diagrams • Force diagrams must include the object and all forces acting on it. • The forces must be attached to the object. • No other vectors may be attached to the object. • Components of forces, axis systems, motion vectors and other objects or surfaces may be included in force diagrams.

  6. FN f = μFN Fapplied = 100 N 10 kg Force Diagram Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate. Weight = Fg = (10 kg)*(9.81m/s2)

  7. To Solve the ProblemNow that you have the force diagram! Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate. FN f = μFN Fapplied = 100 N a 10 kg Weight = Fg = (10 kg)*(9.81m/s2) Write the Newton’s 2nd Law equation for the x- and y- directions. Now plug in what you know and solve for what you don’t. Algebra…YUK!!

  8. Something to keep in mind… • If you are given a Force @ an angle You must determine the “x” and “y” forces independently • Solve for x – axis using: • Fapp COS θ • Solve for y-axis using: • FN SIN θ

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