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Friction in Mechanics: Exploring Causes, Effects, and Solutions

Discover the fascinating world of friction, from its causes - surface roughness and atomic interactions - to its effects on motion, with examples and calculations. Uncover how to change friction, apply Newton's laws, and determine frictional forces using coefficients.

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Friction in Mechanics: Exploring Causes, Effects, and Solutions

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  1. P. 146 30, 33, 34, 35, 37 Homework

  2. What causes friction? • Why is there Friction? Surface roughness Electronic interactions at the atomic level • Friction is caused by the temporary electrostatic bonds created between two objects in contact with one another. • Examples of Friction - Desirable - Undesirable

  3. Examples of Friction - Desirable - Walking - Driving - Braking - Undesirable - Engine Efficiency - Coasting - Pushing a heavy object

  4. Why would I want to change friction? - How would I do it?

  5. Friction & Applying Newton’s 2nd Law System Chapter 6.2

  6. Friction • How does friction affect the motion of objects? • It can slow an object down like the friction between the tires and the road. • It is responsible for increasing the speed of an object like a car. • It is also responsible for objects being able to change direction.

  7. Fforward Ffriction Fnet = FAPPLIED – Ffriction Since the crate is not accelerating, Fnet = 0 FAPPLIED = Ffriction Fground-on-crate FAPPLIED Ffriction System Fgravity Static Friction • Static Friction: • The resistive force that keeps an object from moving. Note: As long as the crate does not move, FAPPLIED = Ffriction

  8. Fforward Ffriction Fnet Fground-on-crate Fforward Ffriction Fnet = FAPPLIED – Ffriction System Fgravity Kinetic Friction • Kinetic Friction: • The resistive force that opposes the relative motion of two contacting surfaces that are moving past one another. • Since the crate will initially accelerate, Fnet > 0. Note: If the crate moves at a constant speed, then FAPPLIED = Ffriction and Fnet = 0.

  9. An Important Term • APPLIED FORCE • Usually whatever is pushing or pulling • NOT the same as Net Force

  10. FN Ff Determining the Frictional Force For people who had a lot of wrong ideas about Physics the Greek alphabet sure gets used a lot! • The force of friction is proportional to the normal force and a proportionality constant ( - pronounced mu) called the coefficient of friction. • For static friction: • 0 < Ff, static<s FN • For kinetic friction: • Ff, kinetic = k FN • Note: FN = the force normal (perpendicular) to the frictional force on the object. •  is dimensionless • Ff, static > Ff, kinetic

  11. Frictional Force For static friction: 0 < Ff, static<s FN For kinetic friction: Ff, kinetic = k FN

  12. Determining the Frictional Force •  (the coefficient of friction) is usually in the range of 0<=  <= 1, but this is not always the case

  13. Determining the Frictional Force Sketch a graph of Fs vs applied force Sketch a graph of Fk versus applied force Sketch a graph showing the transition from Fs to Fk

  14. Ff versus applied force

  15. Ff versus applied force

  16. FN  The Normal Force • The normal force is a force that opposes the Earth’s gravitational attraction and is perpendicular to the surface that an object rests or is moving on. • For a horizontal surface, FN = Fg = mg. • For a surface that is not perpendicular to gravity, FN = Fgcos

  17. FN Fg  The Normal Force FN cos = adj/hyp Fg FN = Fg = mg FN = Fg cos = mg cos

  18. FN FAPPLIED Ff System Fg Example 2: Determining Friction (Balanced Forces) • Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a constant speed of 1 m/s. • How much force is exerted on the crate?

  19. +y FN FN +x FAPPLIED FAPPLIED Ff Ff System Fg Fg Diagram the Problem y-direction: FN = Fg x-direction: Fnet = FAPPLIED - Ff Since the crate is moving with constant speed, a = 0, Fnet = 0, and FAPPLIED = Ff

  20. State the Known and Unknowns • What is known? • Mass (m) = 25 kg • Speed = 1 m/s • Acceleration (a) = 0 m/s2 • k = 0.3 (wood on wood) • What is not known? • FAPPLIED = ?

  21. 0 Perform Calculations • y-direction: • Fg = FN = mg • x-direction: a = 0 • Fnet = Fforward – Ff • FAPPLIED = Ff • FAPPLIED = kFN; FAPPLIED = kmg • FAPPLIED = (0.3)(25 kg)(9.8 m/s2) • FAPPLIED = 74 N

  22. FN Assume Constant speed FAPPLIED Ff System Fg Example 3: Determining Friction (Unbalanced Forces) • Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a speed of 1 m/s with a force of 74 N. • If he doubled the force on the crate, what would the acceleration be?

  23. +y FN +x FAPPLIED Ff System Fg Diagram the Problem FN FAPPLIED Ff Fg y-direction: FN = Fg x-direction: Since a > 0, Fnet = Fforward - Ff

  24. State the Known and Unknowns • What is known? • Force = 148 N • Mass (m) = 25 kg • Speed = 1 m/s • k = 0.3 (wood on wood) • What is not known? • a ?

  25. Perform Calculations • y-direction: • Fg = FN = mg • x-direction: a > 0 • Fnet = FAPPLIED – Ff • ma = FAPPLIED – Ff • ma = FAPPLIED – kmg • a = FAPPLIED – kmg m • a = (148N)/(25kg) – (0.3)(9.8 m/s2) • a = 2.96 m/s2 • Fnet = 148N – 74N • ma = 74N • a = 74N/25kg • a = 2.96 m/s2

  26. Key Ideas • Friction is an opposing force that exists between two bodies. • Friction is proportional to the normal force and the coefficient of friction; static or kinetic. • The force required to overcome static friction is greater than that required to overcome kinetic friction.

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