Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Homework • P. 146 30, 33, 34, 35, 37 What is 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 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 Examples of Friction - Desirable - Walking - - Driving - Braking Undesirable - Engine Efficiency - Coasting - Pushing a heavy object Why would I want to change friction? - How would I do it? Friction & Applying Newton’s 2nd Law System Chapter 6.2 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. Static Friction • Static Friction: – The resistive force that keeps an object from moving. Fground-on-crate Fforward Ffriction Fnet = Fforward – Ffriction Fforward Ffriction System Fgravity Since the crate is not accelerating, Fnet = 0 Fforward = Ffriction Note: As long as the crate does not move, Fforward = Ffriction 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. Fground-on-crate Ffriction Fforward Ffriction System Fgravity Fforward Fnet Fnet = Fforward – Ffriction Note: If the crate moves at a constant speed, then Fforward = Ffriction and Fnet = 0. An Important Term • APPLIED FORCE – Usually whatever is pushing or pulling – NOT the same as Net Force 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. FN For static friction: – 0 < Ff, static < sFN For kinetic friction: Ff – Ff, kinetic = kFN Note: FN = the force normal (perpendicular) to the frictional force on the object. is dimensionless Ff, static > Ff, kinetic Frictional Force • For static friction: – 0 < Ff, static < sFN • For kinetic friction: – Ff, kinetic = kFN Determining the Frictional Force • (the coefficient of friction) is usually in the range of 0<= <= 1, but this is not always the case Material 1 Material 2 Tire, dry Road, dry 1 Tire, wet Road, wet 0.2 Rubber Steel 1.6 Teflon Teflon 0.04 Ice Wood 0.05 Glass Metal 0.5 - 0.7 Chromium Aluminum Chromium Aluminum 0.41 1.3 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 Ff versus applied force Ff versus applied force 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 FN The Normal Force FN Fg FN cos = adj/hyp F g FN = Fg = mg FN = Fg cos = mg cos 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? FN Fforward Ff System Fg Diagram the Problem +y FN Fforward Ff System FN Fg y-direction: FN = Fg x-direction: Fnet = Fforward - Ff Fforward Ff Fg +x Since the crate is moving with constant speed, a = 0, Fnet = 0, and Fforward = Ff State the Known and Unknowns • What is known? o Mass (m) = 25 kg o Speed = 1 m/s o Acceleration (a) = 0 m/s2 o k = 0.2 (wood on wood) • What is not known? o Fforward = ? Perform Calculations • y-direction: o Fg = FN = mg • x-direction: a = 0 0 o Fnet = Fforward – Ff o Fforward o Fforward o Fforward o Fforward = = = = Ff kFN; Fforward = kmg (0.2)(25 kg)(9.8 m/s2) 49 N 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 49 N. • – If he doubled the force on the crate, what would the acceleration be? FN Assume Constant speed Fforward Ff System Fg Diagram the Problem +y FN Fforward Ff System FN Fg Fforward Ff Fg +x y-direction: FN = Fg x-direction: Since a > 0, Fnet = Fforward - Ff State the Known and Unknowns • What is known? o Force = 98 N o Mass (m) = 25 kg o Speed = 1 m/s o k = 0.2 (wood on wood) • What is not known? oa? Perform Calculations • y-direction: o Fg = FN = mg o Fnet = 98N – 48N • x-direction: a > 0 o Fnet = Fforward – Ff o ma = 49N o ma = Fforward – Ff o a = 49N/25kg o ma = Fforward – kmg o a = 2.0 m/s2 o a = Fforward – kmg m o a = (98N)/(25kg) – (0.2)(9.8 m/s2) o a = 2.0 m/s2 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.