What is Friction Why is there Friction Surface

What is Friction? • Why is there Friction? Surface roughness Electronic interactions at the atomic level 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 2 nd 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 = Fforward – Ffriction Note: If the crate moves at a constant speed, then Fforward = Ffriction and Fnet = 0.

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 < s. FN For kinetic friction: Ff – Ff, kinetic = k. FN Note: FN = the force normal (perpendicular) to the frictional force on the object. is dimensionless Ff, static > Ff, kinetic

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 0. 04 Ice Wood 0. 05 Glass Metal 0. 5 - 0. 7 Chromium Aluminum 0. 41 1. 3

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 Fg FN = Fg = mg FN = Fg cos = mg cos

What causes friction? • Friction is caused by the temporary electrostatic bonds created between two objects in contact with one another.

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/s 2 o k = 0. 3 (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 = = Ff k. FN; Fforward = kmg (0. 3)(25 kg)(9. 8 m/s 2) 74 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 74 N. – If he doubled the force on the crate, what would the acceleration be? FN 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 = 148 N o Mass (m) = 25 kg o Speed = 1 m/s o k = 0. 3 (wood on wood) • What is not known? oa?

Perform Calculations • y-direction: o Fg = FN = mg • x-direction: a > 0 o Fnet = Fforward – Ff o ma = Fforward – kmg o a = Fforward – kmg m o a = (148 N)/(25 kg) – (0. 3)(9. 8 m/s 2) o a = 3 m/s 2

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 0. 04 Ice Wood 0. 05 Glass Metal 0. 5 - 0. 7 Chromium Aluminum 0. 41 1. 3

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 < s. FN For kinetic friction: Ff – Ff, kinetic = k. FN Note: FN = the force normal (perpendicular) to the frictional force on the object. is dimensionless Ff, static > Ff, kinetic

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

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.