Net Forces, Friction, Coefficients of Friction

2 Sliding Book Example Why do things not continue to move at constant velocity?Why do things not continue to move at constant velocity? If the sliding book slows down, what’s the force responsible?If the sliding book slows down, what’s the force responsible? –How could I keep it moving at a constant velocity? Why do things not continue to move at constant velocity?Why do things not continue to move at constant velocity? If the sliding book slows down, what’s the force responsible?If the sliding book slows down, what’s the force responsible? –How could I keep it moving at a constant velocity? Friction Friction Remove Friction or continue to add force equal to friction Create a net force of zero

3 The three types of friction 1)Surface or Sliding Friction – Created when an object slides or moves over a second object while their two surfaces are touching. 2)Fluid Friction- Created when an object slides or moves over a second object that is a fluid (Gas or liquid). This reduces the amount of Peaks & Valleys that will interact while their two surfaces are touching. 3)Rolling Friction - Created when an object rolls over a second object. The circular motion constantly refreshes the two surfaces that are coming into contact.

4 Friction opposes Motion Surface Friction Force on person by box Force on floor by box Force on box by floor It’s the sum of all the forces that determines the type of motion. Force on box by person Acceleration What kind of motion is created by an Unbalanced Force?

5 Friction due to the Surface Corrugations in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily. How does the corrugated surface change Friction?

6 Why Doesn’t Gravity Make the Box Fall? Force of Earth acting on Box (weight) Force of Floor acting on Box Force from floor on box and gravity = Net Force of zero. If the floor vanished, the box would fall, Net force  zero

7 What’s Forces are not shown? Force on person by box Force on floor by box Force on box by floor Force on box by person The Normal Force and the Force of Gravity

8 When we drew the box and floor, with the “normal” force and the force of gravity, these weren’t strictly force pairs –forces on the box that result in a Net zero acceleration of the box The real pairs have to involve the earth: When we drew the box and floor, with the “normal” force and the force of gravity, these weren’t strictly force pairs –forces on the box that result in a Net zero acceleration of the box The real pairs have to involve the earth: box floor earth satellite Force Pairs: earth-box (grav) box-floor (contact) earth-satellite (grav)

9 Questions Does friction always exert a force? What does this say about the direction of the frictional force, relative to the velocity vector? What do you think would happen if we loaded lead bricks into the box? Would it become harder to slide? What are some ways to reduce frictional forces? Does friction always exert a force? What does this say about the direction of the frictional force, relative to the velocity vector? What do you think would happen if we loaded lead bricks into the box? Would it become harder to slide? What are some ways to reduce frictional forces? NO Friction vector is opposite in direction of motion Yes Lubrication, change surface, reduce normal force reduce normal force

10 Static and Kinetic Friction Static frictional force: when two surfaces are touching and nothing is sliding, No motion ( ƒ s ) Kinetic frictional force: when two surfaces are touching and at least one of the surfaces are sliding or moving over the other. (ƒ k ) Static frictional forces are always greater than Kinetic Static frictional force: when two surfaces are touching and nothing is sliding, No motion ( ƒ s ) Kinetic frictional force: when two surfaces are touching and at least one of the surfaces are sliding or moving over the other. (ƒ k ) Static frictional forces are always greater than Kinetic

Friction is ƒ =µ N 11 Friction = (Coefficient of Friction) (Normal Force)

12 “Normal” Forces and Frictional Forces Weight of block Decompose Vector Normal Force Friction Force Weight of block Reaction Force From Ramp “Normal” means perpendicular Friction Force = Normal Force  (coefficient of friction) F friction =  F normal

13 Coefficient of Friction Material on Material  s = static friction  k = kinetic friction steel / steel add grease to steel metal / ice brake lining / iron tire / dry pavement tire / wet pavement

14 Stay on the road! What does it take to stay on the road around a curve? –using  s = 0.8 as average for tires on road, F friction = 0.8mg (Normal force is just mg on level surface) F curve = ma curve = mv 2 /r where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp) Got enough friction if F curve < F friction –happens if v 2 < 0.8gr, or v < 20 m/s = 44 m.p.h. What does it take to stay on the road around a curve? –using  s = 0.8 as average for tires on road, F friction = 0.8mg (Normal force is just mg on level surface) F curve = ma curve = mv 2 /r where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp) Got enough friction if F curve < F friction –happens if v 2 < 0.8gr, or v < 20 m/s = 44 m.p.h.