1 / 26

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

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

thayes
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

More Related