What is Friction?

1. What is Friction? Why is there Friction? Surface roughness Electronic interactions at the atomic levelExamples of Friction - Desirable - Undesirable

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

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

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

5. 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.

6. Fforward Ffriction Fnet = Fforward – Ffriction Since the crate is not accelerating, Fnet = 0 Fforward = Ffriction Fground-on-crate Fforward 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, Fforward = Ffriction

7. Fforward Ffriction Fnet Fground-on-crate Fforward Ffriction Fnet = Fforward – 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 Fforward = Ffriction and Fnet = 0.

8. 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<sFN • For kinetic friction: • Ff, kinetic = kFN • Note: FN = the force normal (perpendicular) to the frictional force on the object. •  is dimensionless • Ff, static > Ff, kinetic

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

10. 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

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

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

13. FN Fforward 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?

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

15. State the Known and Unknowns • What is known? • Mass (m) = 25 kg • Speed = 1 m/s • Acceleration (a) = 0 m/s2 • k = 0.2 (wood on wood) • What is not known? • Fforward = ?

16. 0 Perform Calculations • y-direction: • Fg = FN = mg • x-direction: a = 0 • Fnet = Fforward – Ff • Fforward = Ff • Fforward = kFN; Fforward = kmg • Fforward = (0.2)(25 kg)(9.8 m/s2) • Fforward = 49 N

17. FN Fforward 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 49 N. • If he doubled the force on the crate, what would the acceleration be?

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

19. State the Known and Unknowns • What is known? • Force = 98 N • Mass (m) = 25 kg • Speed = 1 m/s • k = 0.2 (wood on wood) • What is not known? • a ?

20. Perform Calculations • y-direction: • Fg = FN = mg • x-direction: a > 0 • Fnet = Fforward – Ff • ma = Fforward – Ff • ma = Fforward – kmg • a = Fforward – kmg m • a = (98N)/(25kg) – (0.2)(9.8 m/s2) • a = 2.0 m/s2

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

22. 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<sFN • For kinetic friction: • Ff, kinetic = kFN • Note: FN = the force normal (perpendicular) to the frictional force on the object. •  is dimensionless • Ff, static > Ff, kinetic

23. 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

24. Ff versus applied force

25. Ff versus applied force

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.