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Friction

Friction. Principles of Physics - Foederer. Friction, F f. Force on objects whose surfaces are in contact Acts in the opposite direction of motion Acts parallel to the surface. F f = μF N F f = force of friction (N) μ = coefficient of friction Unitless (yes, unitless …)

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Friction

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  1. Friction Principles of Physics - Foederer

  2. Friction, Ff • Force on objects whose surfaces are in contact • Acts in the opposite direction of motion • Acts parallel to the surface

  3. Ff= μFNFf = force of friction (N) μ = coefficient of friction • Unitless (yes, unitless…) • Always less than 1 • Describes roughness of surfaces in contact • Rougher surfaces have larger μ

  4. Types of forces acting on systems Fa = applied force FN = normal force = surface reaction force FW = Fg= gravitational force = weight Ff= frictional force = contact force that opposes motion FT = tension = force in rope, string, cord, etc.

  5. Problem Solving Step 1: Draw a diagram with all forces labeled Step 2: Write a net force equation Step 3: Replace Fnetwith ma or 0 Step 4: Substitute and solve

  6. Determine the force necessary to pull a 10 kg box across a horizontal surface with an acceleration of 3 m/s2 if a 25 N frictional force is also acting upon it. Step 1: Draw a diagram with all forces labeled **Everything will have a weight **Friction always opposes motion FN **There is only an applied force if they tell you there is one Ff = 25 N Fa **If the object is on a surface there must be a normal force FW

  7. Determine the force necessary to pull a 10 kg box across a horizontal surface with an acceleration of 3 m/s2 if a 25 N frictional force is also acting upon it. Step 2: Write a net force equation Choose horizontal or vertical FN Fnet= Fa - Ff Ff = 25 N Fa FW

  8. Determine the force necessary to pull a 10 kg box across a horizontal surface with an acceleration of 3 m/s2 if a 25 N frictional force is also acting upon it. Step 3: Replace Fnetwith ma or 0 Fnet= Fa - Ff FN ma = Fa - Ff Ff = 25 N Fa FW

  9. Determine the force necessary to pull a 10 kg box across a horizontal surface with an acceleration of 3 m/s2 if a 25 N frictional force is also acting upon it. Step 4: Substitute and solve Fnet= Fa - Ff ma = Fa - Ff FN Ff = 25 N Fa (10 kg)(3 m/s2) = Fa -25 N 30 N = Fa-25 N +25 N +25 N FW 55 N= Fa

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