Friction

Newton’s 3rd Law You cannot touch without being touchedNewton's third law.

Newton’s 3rd Law • Notice the pair of forces – They are equal and opposite. They act on different objects.

Equal and Opposite Forces • Ouch!!!!!!!

Take a look at this web site to watch an old car crash!!!!

Equal but Opposite Forces • Ouch!!

Notice the difference with crumple zones!!

Friction • Friction is the result of contact between the two surfaces • Is friction good or bad? it depends.

How to Increase Friction • Increase friction by adding sand to the road, add rosin to fingers in baseball, add rosin to the violin bow, a dragster spinning its wheels to heat them up, adding chains or studs to your tires when the road is icy,

How to Decrease Friction • Decrease friction by waxing your skis, coating the frying pan with oil, adding oil to your cars engine, adding water to your water slide.

Friction is a Force • What direction does the frictional force point? • It is always in the opposite direction of the velocity – It opposes the motion.

Friction has Direction • • • • • •

What direction is the frictional force? DEB - a) block sliding down the hill b) block sliding up the hill c) car driving east d) sky diver falling toward the earth e) car going around a curve

Factors which affect FRICTION • 1. the nature of the surface, coefficient of friction, µ − Mu • 2. the weight of the object, W • 3. the normal force, FN

4 Types of FRICTION • • • •

Static Friction Kinetic Friction Rolling Friction Fluid Friction

• Static frictional forces are due to the bonding between surfaces. • The equation that describes the static friction, Ff • Ffs = µs FN

or

F = UN

Static Friction • Static Friction is the force between the two surfaces. • Objects will not move until forces exceed static friction. Another example of Newton’s 3rd Law

Static vs Kinetic Friction

Notice that the applied force is increasing yet no movement

Notice that once F exceeds Fs the block starts to move, accelerates,

and Fk is less than Fs

Coefficient of Friction - µ µs This is the coefficient of friction and is unitless. It is the ratio of Ff : FN

Ff

Ff

Static Friction • 1) If you pull a 40 kg crate with a force of 100 N East and there is no movement, How large is the frictional force and in what direction?

Problem • A large 50 kg wooden crate rests on a stone lab table. The coefficient of static friction between wood and stone is 0.4 • DEB – a) What is the largest force needed to start the crate accelerating?

Surfaces µs

µf

0.80

0.65

rubber on wet concrete 0.60

0.40

wood on wood

0.50

0.20

steel on steel

0.04

0.04

rubber on concrete

Kinetic Friction or Sliding Friction • Slide a block along different sides. Which Force is larger? • F1 = F2 Since N is the same, the frictional force is the same

F1

F2

Static and Kinetic Friction • Friction is not proportional to the velocity or as Velocity increases, Frictional forces remain the same. • Demo- block pull with spring scale. Same value no matter which side is pulled

Sliding Down an Incline • Draw a FBD for a 6 kg block on a 30° incline.

Θ

Sliding Down an Incline • DEB - Draw a FBD for a 6 kg block on a 30 ° incline.

The Sliding Student! Demo - Determine the µs for a student sliding down a wooden board or coin on a book. • Measure the angle when student starts to slide. Draw •

the FBD Θ

Static Friction ∀ µ = tan θ • derivation

Rolling Friction • 3. Rolling Friction is less than sliding friction. This occurs when one object rolls over another. • Ex. Car, roller skates, ball bearings

Fluid Friction • Fluid Friction object in contact with a fluid i.e.. water or air • The magnitude depends upon the speed. V increases, Ff increases

Fluid Friction • So when the fluid friction becomes as large as the weight, ΣF = 0, a = 0, and the object is at constant velocity.

Racing Parachutists • Which parachutist will have greater terminal velocity and get to the ground first?

Terminal Velocity

DEB – Draw the v vs t graph for an object that falls through the air towards the earth.

Terminal Velocity As t increases what happens to the accel?

Terminal Velocity Object

Terminal Velocity

baseball basketball ping pong ball & Raindrop DEB – Predict which has the highest and lowest TV.

Terminal Velocity Object

Terminal Velocity

baseball

43 m/s

basketball

20 m/s

ping pong ball & Raindrop

9 m/s