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Newton’s 3rd Law
You cannot touch without being touched-Newton'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, 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 = s 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 -
s 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
s f
rubber on concrete 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
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 velocityor 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.
Sliding Down an Incline
• DEB - Draw a FBD for a 6 kg block on a 30 ° incline.
The Sliding Student!
• Demo - Determine the s for a
student sliding down a wooden board or coin on a book.
• Measure the angle when student starts to slide.
Draw the FBD
Static Friction
= tan • 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,
F = 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
DEB – Predict which has the highest and lowest TV.
Object Terminal Velocity
baseball
basketball
ping pong ball
& Raindrop
Terminal Velocity
Object Terminal Velocity
baseball 43 m/s
basketball 20 m/s
ping pong ball
& Raindrop
9 m/s